sharepoint online power user playbook is my first book going to launch very soon.
sharepoint online features and benefits
I remembered once faced an error where user having Contribute permission to folder in the library but getting access denied. This issue in entire site collection. User with unique permission to list, folders are not able to view the page. The issue resolved by changing ViewFormPagesLockDown feature to Disable. Below article describes all about the information about ViewFormPagesLockDown feature Unable to setup uniquely secured permission.