As mentioned, this is because there is a convention that all resource-recommendation threads should be made community wiki in their entirety, as agreed on this thread. The reasoning is that the voting pattern on resource-recommendation threads is very different to regular threads, since the votes often follow the quality of the book rather than the quality of the answer, and there is enormous potential for simple answers generating massive amounts of undeserved reputation. (You may or may not agree with this; this thread is not the place to litigate it.)
... and, with that in place, I would argue that the CW label does not fit this thread (since the reasoning for CW'ing does not apply to it), and by extension neither does the resource-recommendations tag nor the accompanying banner. The question deals with books, yes, but it is not asking for recommendations of resources.
As to whether the question is on-topic -- yes, it is a subjective question, but I think it makes the bar in terms of objectivity and should maybe be allowed to remain open. The general standard here comes from the Good Subjective, Bad Subjective SE blog post, and this thread has already shown that it admits answers based on expertise that pull together a large body of objective fact into a useful, informative, actionable, verifiable opinion.