I think the second is correct as present perfect is used in the main clause.
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Anonymous have never found a book that was worth reading.I have never found a book that is worth reading.All I can add is that both forms are very common. Since present perfect ties the past to the present, both present and past tense in the dependent clause are quite natural and carry the same intent.
Anonymous What is difference between these sentences?You can use either one. It doesn't matter whether you think of books as they were found, one by one, during the time of your search (was), or you think of them as they exist as permanent records (is).