If you want to learn SOQL advance concepts, make use of the below link
http://www.packtpub.com/getting-started-with-soql/book
http://www.packtpub.com/getting-started-with-soql/book
This book will be very helpful for DEV 501 certification preparation.
Note: Kindly refer this to your friends who are interested and preparing for Salesforce certification.
Cheers!!!