Absolute Magic and Pure Effect are two of my favorite books, but unless you get lucky you're going to be spending 200-400 to get both.
I really enjoyed Maximum Entertainment by Ken Weber. It's a no-nonsense, to the point book about making a performance better.
Scripting Magic 1 & 2 are great. This is an excellent example of a book with a clear goal that doesn't get muddied up. Pete aims to help people write better scripts, and both volumes do just that - teach you how to write better scripts. No fluff.
One of the more obscure volumes I really enjoy is Alchemical Tools by Paul Brook. Maybe it's because he's a mentalist and advertises more to that market than general magic, but I really enjoyed that book and it really made me think about how I was performing.
Counting Scripting Magic as one set, those are my top 5.
Strong Magic is good, but I had issues with it. For one, I don't like how often Ortiz references his other works. It comes across as both bragging and a sales pitch to me. I also felt that whenever he strayed out of his specialty (As in, when talking about mentalism or bizarre magic, for example) he was clearly not drawing on personal knowledge. I think everyone should read this for sure, but it does have flaws.
Jamie Ian Swiss has several books out, and there was recently a nice box-set of leather bound versions. I think they called it "The Works". I read 3 of 4 of them so far, but got burned out on Swiss' writing style a bit. Jamie has a reputation as the controversial "bad boy", and when you read his stuff you can tell that's something he was clearly going for.
Swiss and (to a lesser degree) Ortiz also are part of the cynics who call themselves skeptics. This is relevant to the thread, because their anti-mystic views interfere with their own goals. Swiss talks a lot about creating miracle situations for audiences, but then focuses on meaningless card tricks. They are so opposed to anything that might possibly hint at the supernatural that they don't allow their audiences to enter that space of possibility. I've always felt the need to tell the audience that it's all tricks and sleight of hand to be utter arrogance, though, so feel free to take my opinions with a grain of salt.
Our Magic is kind of the original Theory Book. I enjoyed reading it quite a bit - particularly since what they talk about is still relevant, today. Change the wording around a little bit and their essays would look right at home on any contemporary magic forum. It may help that I scored a first edition print and I'm a huge book nerd.
Another book that has a good bit of theory in it is The Paper Engine by Aaron Fisher.