I hate it when you ask people the best book they have ever read and the first thing they say is harry potter without thinking.
If i have to answer a question like that then i will have to consider how much inspiring the book was.
One of the most inspiring book i ever read was "The Strange case of Dr. Jekyll and Mr. Hyde" (Its a book written like about 100 years ago roughy)
A fiction about a happy doctor who performed experiments that affected his life and turning him into an inhuman and wicked creature. the settings of this story was based in Old London.
I also love crime fiction and investigation. Very interesting!
The Left Behind series by Dr. Tim Lahaye are also a very interesting books. There are about 12 volumes and still more are being written.
Dont forget to read books by Rudyard Kipling A Indian born British who writes interesting fiction books based on animal characters such as mowglis brothers and Riki Tiki Tavi.
Multicultural fiction based on Native american history and the Gold Rush Era and even the Oral Traditions are really fascinating.
Try books with all of these. Its like travelling back in time and Good luck.