I have asked a similar question here on meta:

 - [Why are book recommendations off topic, but software recommendations on topic?](https://music.meta.stackexchange.com/questions/878/why-are-book-recommendations-off-topic-but-software-recommendations-on-topic)

were, a mod (Dr. Mayhem) answered:

> [...] software recs are on topic, but that some can be, **if the question is written well**. I think book recommendations should be treated similarly to software recommendations. 

>  [...] but as there is this uncertainty[...]

From what I remember, there have been some well written questions that were allowed, because they simply weren't of the format 

> suggest me some books for X!

but, they included research etc.

But, generally, these kind of questions get closed, because every user has something different to suggest; thus it is opinion based; thus off topic.

The Chat is a good place to discuss these things.