It seems that Amazon's search algorithms share Peter Woit's opinion of string theory - that it's not physics. At least I could not connect by searching the physics books for "string theory." They do, of course, carry a number of books on string theory, though, to be sure, many of these are about violins and cellos.
Perhaps some of you doubt that there are a hundred books on string theory. You would be wrong.
Others would doubt that I am qualified to properly evaluate string theory books. They, of course, would be entirely correct. But I won't let that bother me. My analysis will be a bit circumscribed, since I intend to stick to books that I have actually opened, i.e., mainly those I own.
There should really be two or three lists, since the levels of the books considered are very various, from popularizations to monographs and high level textbooks, but I will merge those too, but will include a letter designation - P for popular, I for intermediate level books, and H for the high level textbooks.
It's customary to arrange this sort of list from worst to first, but I'm too lazy to do that, so I will just start with number:
1. The Elegant Universe(P), by Brian Greene. Greene set out to explain string theory "to the stupid people," as an uncharitable colleague put it, and does a terrific job. This book is a masterpiece of science popularization and did a lot to make string theory famous. Recommended to almost everyone - even smart people.
2. A First Course in String Theory, by Barton Zwiebach (I) This is a technical book, but written so as to be accessible to bright undergraduates - all you really need is some quantum mechanics, relativity, and a slight familiarity with a Lagrangian. Also excellent for physicists who have forgotten a lot of quantum field theory. I really like this book.
3. String Theory and M-Theory (H) by the Becker Sisters and John Schwarz. Modern and up-to-date (2007). I like the cover.
4. Superstrings: A Theory of Everything? (P) by Davies and Brown. An oldie but a goodie from the vigorous adolescence of string theory. Some explanatory material plus short Q&A with some string gods (Witten, Schwarz, Green) and some Nobel Prize winning doubters (Glashow & Feynman) and a more nuanced appreciation from (NP winner) Abdus Salam and others. Recommended to all.
5. Superstring Theory by Green, Schwarz, and Witten. (two volumes) (H). The monograph documenting the first string revolution. Old (1988) but beautifully written.
6. String Theory (H) (two volumes) by Polchinski. Joe's big book of string (1998). Written by one of the guys involved in string revolution numero dos, but doesn't get into M-Theory. Currently available in dirt cheap paperback from Amazon ($74.88 for both).
7. Not Even Wrong (P) Peter Woit's anti-string book. Written at a much more popular level than his blog of the same name.
... (maybe I will add some others later)