When talking with patrons about books, I try my best to keep the conversation to book characteristics (this book is engaging and fast paced) rather than whether or not I liked it. This post from Book Riot covers most of the “best practice guidelines for bookish interactions” that I follow here in the library:
Why Library Staff Don’t (or Shouldn’t) Comment on Your Library Checkouts
I don’t necessarily agree with everything, word for word, but I think that it’s an interesting piece, and worth a read!