In contexts such as OP's, X-wise means when considered specifically in the context of X. It's a construction commonly used to create "nonce" terms (one-off coinages whose precise meaning is specific to the context, but normally obvious to the reader/audience in that context).
Here are a couple of instances from Google Books for dinner-wise (just a noun I chose at random). Note that they're separated by over 150 years, showing that the construction has been around a long time with no significant change in how it can be used...
Gentlemen, I incline not dinner-wise. And why? — I have dined already
(Brother Jonathan, 1842)
I don't know what the wife can do dinner-wise but there'll be something
(Death at Hallows End, 2008)
Perhaps unwisely, OP's cited context blurs the distinction between intelligence and knowledge. Almost certainly the quiz-setter is asking whether you think you're good at book learning. That's to say, are you good at absorbing information from books (rather than personal experience), and/or good at passing exams and succeeding in an academic context (as opposed to having practical skills, common sense, etc., being types of "intelligence" often claimed by "non-academics").
In OP's specific case, book-wise primarily restricts the context of "intelligence" to the ability to absorb information from (academic) textbooks. But it's easy to imagine a context where the restriction is significantly different...
I get through several tabloid newspapers and magazines every day. But book-wise I'm not much of a reader, so I hardly ever visit the local library.
...where in that context the speaker may well be referring to books as a source of entertainment (novels and other works of fiction), rather than "information" as such.
EDIT: I've just realised that some learners may find OP's specific example confusing because they might understand intelligent as meaning wise (having wisdom). But etymologically that word is unrelated to the suffix usage. From Your Dictionary...
The suffix -wise has a long history of use to mean “in the manner or direction of,” as in clockwise, otherwise, and slantwise. Since the 1930s, however, the suffix has been widely used in the vaguer sense of “with respect to,” as in
This has not been a good year saleswise.
Taxwise, it is an unattractive arrangement.
Best Answer
It is referring to the assumed 'clandestine event' mentioned in the previous sentence. Isabel is worried that her response:
will lead Rock to assume that there were interactions between Isabel and Lord Nicholas. Then she hints to the reader that those assumptions about their interactions would not be wrong. You could read that last sentence like this: