The most common and most likely answer to this crossword clue is the 11 letter word THESAURUSES
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Buddy explains!
The straight part
We know that "books" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case books = thesauruses.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does THESAURUSES mean?
THESAURUSES noun- A publication, usually in the form of a book, that provides synonyms (and sometimes antonyms) for the words of a given language.
- A dictionary or encyclopedia.
- A hierarchy of subject headings — canonic titles of themes and topics, the titles serving as search keys.