Among Others by Jo Walton went straight onto my favourites shelf! Or it would have done, if it hadn't been a Kindle book, so instead it went into my favourites folder.
It's hard to quantify what makes a book become a favourite. This book had so much about it that I loved. The main character was, like I was at the same age (and still am), a voracious reader. She devours the contents of bookshelves and libraries just like I did, and she loves science fiction and fantasy in particular. There are dozens of references to other books within this book, many of which I'd already read, but many others that I now want to read. In fact, I'll probably read the book again just to pick out those books I particularly liked the sound of. And I've also pulled out my Waterstone's Guide to Science Fiction, Fantasy & Horror to have a browse through.
There's also the boarding school setting - so many of the books I've loved over the years have been set in schools (particularly boarding schools) or universities. I think it's the idea of lots of people being thrust together and having to make friends that appeals to me.
And, best of all, there's the magical element. All my favourite fantasy novels are set in our own normal world, with just a sprinkling of something strange and magical, from Charlotte Sometimes which I read as a child, to Tam Lin which I read as an adult (and which both live on my favourites shelf). I think this book has a lot in common with Tam Lin (by Pamela Dean) in particular. In both books there's the listing of the sci-fi books the character owns or reads, an educational setting, and the existence of fairies in our world.