I don't think the books are canon but first off Garak is truly one of my favorite characters and second off Andrew Robinson wrote an amazing book about him that I highly suggest you read if you want to know more about him. He's complicated but is absolutely not a good guy, at least not in the traditional sense.

The book is very good! It's called A Stitch in Time and apparently the audiobook is narrated by Andrew Robinson. I read the book, mostly in his voice, but now that I know the audiobook was narrated by him I need to get it haha!