I know it's not a book but there is a good (and inexpensive course) you can take on Udemy.com that teaches a LOT about Odoo 9. Everything from installation to development.

The documentation effort has been focused online rather than for books:

For a User audience - https://www.odoo.com/documentation/user/9.0/

For a Developer and IT Audience - https://www.odoo.com/documentation/9.0/

For books, I do know of a project to update https://essentials.odoo.com/page/homepage to version 9.