The most common and most likely answer to this crossword clue is the 9 letter word ANTINOVEL
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "experimental book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case experimental book = antinovel.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 ANTINOVEL mean?
ANTINOVEL noun- A novel that deliberately avoids the typical conventions of the novel, such as a coherent plot and protagonist.
- (by extension) Any style of writing that deviates from the norm of technical conventions used in writing literature.