You have to be PMI member to access this, please go to below link and click on 'Go to eReads & Reference'.

http://www.pmi.org/Knowledge-Center/Virtual-Library-eReads-and-Reference.aspx

They are little easy compared to PMI, covers mostly PMBOK and many of them of direct sentences from PMBOK.  I got many questions from PMBOK directly in my final PMP exam, so that way these questions are useful.

Regards