On money.se, we get a lot of product recommendations in the form of links from new users who have never posted anything else. The recommendations generally are valid answers to the question, at least in theory.
Chances are most of them are spam in the sense that the user is just promoting something they are involved with or posting an affiliate link, but it's not generally completely clear-cut. I tend to just delete those answers but clear any spam flags, and warn the user about self promotion.
But this case is quite different: the user has 6K rep so must have posted plenty of other good content. High reputation isn't in itself proof that the user isn't a spammer, but since most of our spam is drive-by it does at least mean that it doesn't fit the usual pattern.
Looking at the detail, it's rather unlikely that the book recommendation is self promotion. I agree it isn't a very good answer, but it doesn't feel at all like spam.