I can't say enough good things about this ebook. It may seem brief (56 pages) but it's extremely concise so you don't have to sift through pages and pages of meaningless banter.
I can honestly say that The Lazy Dungeon Master has completely changed my career as a GM for the better. While I used to spend hours preparing a session, causing unnecessary railroading, I can now prepare a session in 15-30 minutes, all thanks to this book (and A Fistful of Dice's follow-up video on reducing prep time).
Seriously, GMs, if you buy one resource book, make it this one. It's worth every penny. It even prints out very nice and crisp!