I need help researching purchases made in a prior year using QB 2015 Pro. QB automatically closes previous years' books and I don't know how to get around that.

You should be able to run reports for previous years - even open specific transactions (for example, a check or deposit). QB will prevent you from changing previous year transactions unless you have a password (which is a good thing since you don't want to change history since tax returns are based on prior year transactions.)