I like to think I'm a fine connoisseur of DM's guides, but frankly, I read mostly internet stuff about it. In other words, my main guides are the specific ideas for dealing with specific issues that arise at the table, because blogs often address these ideas and have good responses. OSR-style blogs are better, because their DM advice consists of things like new plot hooks, not railroading players, and other ideas that don't depend on answering rules questions (I first got back into D&D during 4e, and those blogs were mostly working around that bad ruleset to have a good time: less useful).
But recently I have read two actual books about DMing - the Lamentations of the Flame Princess Referee Guide [Grindhouse Edition] (hallelujah for this being available for free on the internet while we await the latest Ref Guide Mr. Raggi will put out) and Alexis Smolensk's How to Run. Both are opinionated, in an informed and positive way for the most part, although their hard-headedness leads them to the signature flaws in both books. That said, both contain worthwhile ideas and would actually form excellent basic guides for running a D&D game, which is an extremely demanding task.
Obtain Mr. Raggi's book first upon thinking you'll ever run a D&D game. All of the advice is useful and succinct, and he explains to you in a style that is a pleasure to read all of the basic principles for running a game. I like that the Ref's Guide is oriented towards the beginner, but I enjoyed it because it also contains numerous 'first principles' that will always be useful to keep in mind when running games. Many may complain about the lack of monsters and numerous magic items, but I actually like how these were handled - guidelines about how to create types of monsters, including beast or animal stats with sizes leading to likely HD, and guidelines about how disruptive magic items can be. For all games you want to run that aren't about fantasy superheroes, an excellent set of suggestions.
Mr. Raggi's Ref's Guide also benefits by being mostly about the game - advice about dealing with players is minimal (mostly in the variety of don't put up with bitches/assholes, which is true and enough said at that point). These days, the advice about dealing with the social aspect of DMing has taken up way too much space. Sorry guys, learning to deal with people and friends and the contentions that arise in playing a fucking game with a group of other human beings is not a RPG-specific skill. Get it out of my game books - there's so much specifically difficult about running the King of Games that I want covered and to have helpful tools to address, I don't want energy taken up with RPG-versions of etiquette and advice columns. Including this in RPG books is like including advice for how to operate a computer (or excel) or how to do math - useful skills that overlap, but are basically not RPG specific.
So anyway, obtain the Ref Book (buy some LOTFP to support Raggi) and read, incorporate it's ideas into your game if you haven't already. He explains how to have a non-shit game.
My only complaints about the Ref Book are basically that it still heavily relies on the DM having found and purchased premade adventures, despite also having some good sandbox/worldbuilding advice generally. In other words, what it really lacks is more generators - more example random encounter charts, NPC charts (it does have a basic one), hex-crawl generators, etc. To be fair, of course these materials are obtained elsewhere easily - from LOTFP - like Vornheim, Yoon-Suin, etc. And this edition of the Ref's Guide is part of a box set that had an example hex-crawl, adventure, and so on. But more tools for long-term play with some concrete "seeds" or generators would have taken this book from "useful" to the indispensable replacement for all DM guides (and this is something no monster or treasure list need replace).
Also, that said, I love that LOTFP in general has replaced the old B/X learning pipeline - if you know from your internet reading how to do this. Get your free rules and free DM's guide, download Better than Any Man sandbox, and then use the DM's guide and other LOTFP adventures to move on from there. You can also then go to Vornheim, and for a higher-level game, a Red & Pleasant Land. But, Mr. Raggi, PLEASE POST THIS TO YOUR WEBSITE in some clear and understandable manner. Let's get the guy who is fine with D&D 5 but vaguely dissatisfied and may want to try his hand on DMing, but doesn't have tons of experience, a solid pipeline without having to read blogs for months and years.
Ok, Smolensk's book How to Run. This book, basically like the dude's blog, has some extremely good diamonds inside, but is, to say the least, hidden in the rough. As everybody knows, the dude bloviates, and sorry, it's not some sign of intelligence, it's just useless bullshit. Basically, his style, while also readable, is far worse than Raggi's and I found myself skimming page after page of prefatory bloviating to get to paragraphs with the advice. I'm glad you write man, but EDIT your work. Cut the fat from your writing. This is no problem on your blog (it's a blog), but it's annoying in your book. Smolensk even seems to realize it because he includes helpful bullet-point conclusions at the end of every chapter with his main advice.
The good and great thing about How to Run, which is shares with Raggi's book, is that it makes explicit what I'll call "consequences" DMing as the best way to run. Adhere to a coherent logic about the consequences of the player's actions, add conflict, be fair about the consequences, add conflict, and you're game's "adventures" will write themselves (although, as Raggi well-says, you need to populate your world with plenty of hooks, meaty adventures and 'dungeons' too). All of Smolensk's advice on this count is well taken and well explained - it helped me improve my game. There's good ideas and advice in here about how to do this and how to be fair. It is truly an advanced guide, in that it assumes the DM has been at it for awhile, and so has excellent suggestions here and there for the type of little problems that often arise at the table. This includes giving some "tough love" about frequent DM foibles - gamer ADD, changing your ruleset, changing your world - all flaws I've exhibited. [A niggling point here - maybe some advice about when your current line of adventures is boring or fallen into a weird rut, how to change tack without switching games? The book has other stuff touching on this you could read about, but nothing direct.] Definitely a worthwhile read if you frequently run a game.
Now, I doubt Mr. Smolensk realizes how to cut the fat from his writing in How to Run, because his book is filled with exactly the sort of 'social' advice I lament above. Page after page about his stellar abilities in reading people and social situations, the importance of adrenaline/social tension, shit about dilating pupils, and so on. It's almost farcical. Look, social dynamics are an important facet in running a successful game, but social skills are simply a part of life. Please spare me your 'war stories' about all the weird social situations you've dealt with - they're nothing special. The pop psychologizing about the social dynamic at the table, with it's explanation of stress is also facile. Remove this from your book, it does nothing to convey good advice about DMing. Maybe this is Mr. Smolensk's secret charm, like Gygax's interests in polearms and harlots, but it's not for me. I would suggest in the next book or edition for Smolensk to remove all of this, or better yet: just keep the good gems and please stop thinking of DM advice including some sort of psychological treatise. E.g. (1) a good and comfortable table and chairs are overlooked but important in putting everyone in a good gaming mood, (2) sometimes players and the DM will get excited and intense about the game, this is good and natural, but try not to let it lead into outside the game fights, (3) everyone needs to handle stuff like an adult by directly talking about things-if a player can't handle that, he can be kicked out from the game, or however Smolensk would characterize this stuff. Maybe this is a little cruel, as I'm basically advocating that this book would be better if it were written in Raggi's more concise and clear style, so take my criticism with a grain of salt. Sorry man, you're just not that special about handling social situations - please stop talking about it so much and give me more concrete advice about running the game. Maybe this is an organizational issue, and the social stuff could be confined to a single chapter for those who liked it (J.B. at BX Blackrazor).
Again, in any case, both books are worth getting. Mr. Raggi's moreso and from the beginning of a DM's running games, while Smolensk's is more of an acquired taste that's useful after you've been running a little while. I like very much that both distill advice that you'd have to read hundreds of blog entries to otherwise acquire, and reflect some of the best contemporary thoughts on RPG DMing.