I don't think it is clear whether it should be "was" or "were". My personal reaction as an English speaker is that it should be "were". The subject of the sentence is "what I read", which is plural because it refers to more than one thing. In fact, that plural reference is what the sentence asserts.
Your example is a pseudo-cleft sentence, and such sentences are discussed briefly by McCawley on pages 64-66. What I have just said seems consistent with his partial analysis. See pseudo-clefts in Syntactic Phenomena.
On the other hand, perhaps the example can be construed as stating the subject matter of the books I read, and if "subject matter" is the referent of the subject, perhaps the main verb should be "was".