I'll third the Raging Swam Dungeon Dressing books because they are that awesome.
From Goodman Games:
-Grimtooth's Ultimate Traps Collection. It is coming to retail soon (once the KS backers get their copies). It's systemless, so GM's can set their own DC's for the traps. Also, it's a hoot to read!
-GM Gems. This is a book of tables and essays written by GM's for other GMs centering on three main factors: Urban Adventures, Wilderness Travel and Dungeons. The mechanics in this book are for Dungeon Crawl Classics, but there's not much mechanics in here, it's mostly ideas and strategies.
-Monster Alphabet: This book is a series of tables from A to Z that gives you ideas to modify monsters. Again the mechanics are built around Dungeon Crawl Classics, but everything they mention can be converted from the Bestiaries (or just made up on the fly).
Frog God Games:
-Bill Webb's Book of Dirty Tricks: This is a controversial one. If you do all of what he recommends all at once to your players, you might lose some or all of them. BUT: he's a long time GM with good advice on clever ways to deal with some problems that GM's will face.