What is the best css book for beginners?

Instead of books..,

go to www.w3schools.com/css and check the basics..

if u need book then 

Book Name: Beginning CSS Web Development: From Novice to Professional

Author: Simon Collison

Publication: Apress 

Learning Web Design: A Beginner’s Guide to HTML, CSS, JavaScript, and Web Graphics

by Jennifer Niederst Robbins

i think it is good start ,

HTML and CSS: Design and Build Web

Best CSS book for beginners is

HTML-CSS-Design-Build-Websites

it contains alot of illustrations and exmaples, it is focus on practice

also you can start learn and practice CSS for free from

http://www.w3schools.com/css/default.asp

also you can get certificate if you pass test exam

don't refer books instead refer http://www.w3schools.com/css/