" the so-called classics" suggests that there is a (broadly) fixed or defined set of books that are generally agreed to be "classics". Without "the", the "fixed set" idea is not present (or not so strongly, anyway). It just means any books that people call classics.
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"the so-called classics" suggests that there is a (broadly) fixed or defined set of books that are generally agreed to be "classics". Without "the", the "fixed set" idea is not present (or not so strongly, anyway). It just means any books that people call classics.