Usually, I don't really ever regret reading a book later than I originally planned. When I read it, I read it and that's fine with me. Just as long as I read it, it's all good.
If there is one book that I probably will regret not having finished sooner, it will probably be My Ishmael. I started reading this book after getting it soon after reading Ishmael. I own it and have it somewhere, I just haven't finished.
I'm sure the book will be amazing. The first one was. It blew my mind.