There are (with one outlier that we will come to in a minute) only two broad categories of books on the subject of string theory. The first are the ones written for popular consumption, which generally assume absolutely no knowledge of physics whatsoever on the part of the reader. Here, of course, I am speaking of books like The Elegant Universe by Brian Greene, or Hyperspace by Michio Kaku. These books are good, by and large, at informing the general public about the subject at hand. Many of my peers claim to find them overly sensational in some aspects (and it is often hard to argue with that), but the way that I figure it, these books constitute our “marketing” wing; the more public interest there is in the subject, the more funding we will be able to get. They are, however, almost completely non-mathematical, and so for the purposes of more deeply understanding the subject at hand (or persuing original research in the field), they are quite useless.
The second category of books are those written by string theorists for string theorists; even though they have names like Introduction to String Theory, they assume at least a Master’s level knowledge of Quantum Field Theory, and launch into fairly impenetrable jargon with very little explanation almost immediately. They are, in short, fairly unhelpful to anyone who is trying to use them to teach themselves string theory (rather than just using them as, for example, supplementary readings in a course).
The sole exception to this rule (at least in so far as I have seen) is Barton Zwiebach’s A First Course in String Theory, which assumes only an unergraduate-level grasp of quantum mechanics and general relativity. I am not a string theorist, but my research area is an application of string theory, so this is the ideal book for me to use. It will therefore come as absolutely no surprise to any of you to hear that such a useful book has consistently been checked-out of every academic library I have visited.