Recommendations are fine in answers if they arise naturally as part of answering the question.
The perceived problem with asking for recommendations is that they easily fall into "big list" type of answers which are difficult to keep up-to-date, hard to vet for quality, and just generally fall apart and become pretty useless. There may be some site that still allow them, but IME as the site grows they get shut down.
If you have book recommendations, then I think the tag wiki pages are an excellent place to put them. (Click on a tag, then click on "learn more...".)
After the link in the question was pointed out to me, I see that I upvoted and starred a question which boldly asks for book recommendations. So, my personal feelings clearly are not as strong as my answer purports. It seems that it can be done, but you have to be creative about it. Some context and structure and motivation for why recommendations here are preferable to a google search would seem to be required to let it fly. But I suppose it's not a blanket prohibition.