Although the LEED reference guides should be used when studying, going through all 500+ pages can be a daunting task. This is why I recommend using a study guide to supplement your studying.
The following guides will help you better gain an understanding of the material you need to know for your particular LEED exam. Click on the one that interests you for more information.
The following third party study guides are highly recommended by greenexamacademy.com.
In addition, the USGBC will be offering study guides and other exam prep material soon. For more information about that, you can check out this here.