1. Book Library Pro can be installed on Joomla 2.5 without any functional limitations
2. Joomla 3.3 over Joomla 2.5: it is not connect with our component, it works well on Joomla 3.3 and Joomla 2.5, it changes in Joomla CMS. More details you can find on Joomla.org
If you have other questions, please ask.