I don't go to the library very often. But when I do, I always try to get recommendations from the librarians on new books to read. While some have not been that great, most of them turn out to be some of my favorites. Since most of the time, at least one of the librarians has read the book (especially if it is new) before it goes on the shelf, they are able to easily recommend a favorite.
On the other hand, if a book is new or I already know that I like it, I am more likely to go and buy it especially if it is a popular book. I am not very patient when it comes to my book series and when a new book in the series has come out, I want to read it now, not wait for two months to even start it (which happens when libraries only have one or two copies).