The most common and most likely answer to this crossword clue is the 8 letter word RESERVES
I've seen this crossword clue in The Times
R
E
S
E
R
V
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S
Buddy explains!
The straight part
We know that "Books/that are used only when needed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case books/that are used only when needed = reserves.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 RESERVES mean?
RESERVES noun- (behaviour) Restriction.
- That which is reserved or kept back, as for future use.
- (social) Something initially kept back for later use in a recreation.
- In exhibitions, a distinction indicating that the recipient will get a prize in the event of another person being disqualified.
- (calico printing) A resist.
- A preparation used on an object being electroplated to fix the limits of the deposit.
- To keep back; to retain.
- To keep in store for future or special use.
- To book in advance; to make a reservation.
- To make an exception of; to except.