So many books to choose from!
I go through phases of having favourites, but consistently at the top is The Catch Trap by Marion Zimmer Bradley.
It's about a young circus boy who discovers that he has a talent for the trapeze, then he falls in love and the book is about a doomed love affair really, but not in a mills & boony way.
When I am asked about my favourite books, I tend to generalise and say "anything by Terry Pratchett or Dean Koontz" and, whilst I like Marion Zimmer Bradley, I would never list her as one of my top authors, yet this one book just always does it for me. Every time I read it.