Which sentence is correct? What you need is books. What you need are books.
I can see the confusion of this sentence. Subject/verb agreement is at the heart of the issue.
In order to simplify things, what you can do is rearrange the sentence. This simple act, sometimes makes things much easier. Moreover, we can use this technique in many sentences.
"Books is what you need."
"Books are what you need."
When you rearrange the sentence in such a way, it becomes clearer that the second option is correct. The reason for this is because the word "books" is plural, and for this reason the verb needs to agree.