I like that this is community wiki, so I can throw out a crazy idea and not feel bad if people vote it down. I've come across a number of questions on MO of the flavor "please give me a textbook recommendation for subject X." What about having a page in the database for mathematicians to rank textbooks in various fields, e.g. by voting for them.
The obvious problem I foresee with this is that different textbooks are good for different levels of learning, but I suppose this page could be geared towards undergrads or early grad students who don't yet know which books will serve them best. By the time you are an expert in a field you don't need textbook recommendations, so this aspect of the database would need to be purely pedagogical and geared towards people starting in a subject.
As it stands, I need to search several websites (e.g. Google books, Amazon, Goodreads, etc) for recommendations and these can come from literally anyone. With this database I assume we'd be restricting who can post to the database so that only professional mathematicians could. So I would know that I could trust the recommendations as coming from a mature and knowledgeable source rather than an undergrad who just got a bad grade and blames the textbook.
