Sorry for jumping in late but this is what I think about this whole thread. First of all, books on theory are not like a book on sleights. You can not master theory by reading it and practicing it in your bedroom in front of a mirror. You have to go out and perform it, much like Cold Reading. As far as theory goes you can read the same book 5 times and learn something new from it every time you read it.
Next, I would not buy more than 2 books at a time. If you buy too many you will over load yourself and confuse what you are reading with other books you are reading. Take one book and study it over the course of a few months.
You asked for books on theory, so, not knowing anything about the book, would you buy Erdnase when you are wanting books on theory. You can get Erdnase for free (legally) from just about every magic site out there. So why waste the money until you know whether you will like it or not. Erdnase is not for everyone. I say download the Erdnase ebook and read through it first before buying it.
Now on to books on theory.
From the list you gave I would go for Designing Miracles (first) and Strong Magic (second). But. Strong Magic is the type of book that you should seriously study only if you are planning on being a serious working magician, not a kid doing magic for friends and family. Now, that being said, I am not saying you only perform for friends and family or are not planning on being a "worker". I am only saying if you are not planning on working don't waste your time with Strong Magic.
Designing Miracles on the other hand I believe is an amazing book on theory that EVERY magician that plans on creating his own effects should read. It covers everything you could possibly imagine. I would buy this book first.
One problem you are going to come across with reading all these books on theory is a very BIG problem. They are all theory on magic according to the person that wrote the book. That is a big reason why some people don't like the Fitzkee Trilogy. They don't like his theories on magic. So, what I am trying to say is reading a ton of theory books might not be best for you. Stay with one author at a time because they might contradict each other.
WHERE IN THE EFF ARE YOU GETTING YOUR PRICES!?
I bought both Designing Miracles and Strong Magic at the same time for a little over $60 for both books together. Before you buy anything, I would do a little more shopping around than the quick Google search you did to get your prices. What I would do is go to
http://murphysmagiccommunity.com/profiles/friend/listFeatured and shop around in those shops for the best deal. Naturally, I am going to suggest going to ExoMagic. You will get the best deal from them by far, but their shop is down for the time being.
I am putting together a bar and restaurant routine so I am looking for theory on Bar and Restaurant magic. I bought Tangled Web by Eric Mead a week ago and it seems to be a pretty good book on both effects and theory. It covers walk around and bar magic. It is written more as a conversational book and not as a teaching manual which makes the read go that much more smooth. I am also planning on buying David Stone's book Close Up The Real Secrets of Magic. I understand it is full of theory on getting jobs as a restaurant magician.
So, what I am saying is :
Decide whether you are going to be a worker or just performer.
Shop around before you buy anything.
If you are going to be a worker, figure out what type of magic you want to perform and buy your books accordingly.
Sorry for the rant,
Sam