I recently noticed the book-recommendation tag and I've been wondering why it isn't a synonym for the reference-request tag. All I could find here on Meta is
that it used to be so when it was created, but I couldn't find why it was made separate from reference-request.
I agree with Martin Sleziak, so I'm leaving this question open to allow further discussion. I posted a proposal to merge books and textbooks with book-recommendation here, though.
Edit2: A quick search shows that only about two thirds of the questions currently tagged reference-request are about books. Therefore I'm dropping the proposal for merging book-recommendation with it and I'm accepting AlexR's answer.