The required reading differs quite substantially from subject to subject. A large number of resources are in electronic form and are provided by the lecturers who may upload them to the course pages in Moodle. You can find individual teachers’ requirements in the SIS in the Literature field. Textbooks are usually available from the relevant libraries, and can often be borrowed for study purposes for a full semester.
Students usually share a lot of study material. However, it is always necessary to take into consideration copyrights and the quality (or lack thereof) of the material obtained from unverified sources. Closed study forums are popular: Bioforum (bioforum.cz; Czech only) and Geografové (geografove.cz; Czech only), as are accounts whose names include “Biosoubory” at ulozto.cz and Google Disk (Czech only).