It'd be helpful to get an idea of the systems you will be working with (C&S and obturation). Ask your program director or current residents. If you have the time, practice these techniques on extracted teeth. ie. finding MB2, warm vertical obturation, etc. If you have access to a microscope, it'd be helpful to get acclimated to it (positioning of patient/yourself, adjusting mag, etc). There's a high learning curve with all these techniques and it takes time.
Pathways of Pulp is the endo bible, but not necessary to commit to hard memory until residency starts.
Like Boat mentioned, loupes are useful if you're teaching pre-doc or moonlighting. I don't know about other programs, but we exclusively use microscopes in our graduate clinic for patient treatment from the start. I've heard of residents at other programs that use loupes in the beginning, but only because they weren't comfortable with microscopes or ready to part with their loupes. Most end up transitioning to microscopes due to the complexity of the cases.
Get in touch with your local endodontists/mentors and ask them for advice as well. For example, one of my mentors specifically mentioned to seek out Thermafil retreatment cases in residency as our metroplex has a high number of those patients.
With that said, enjoy the time you have right now. Travel, hang out with family/friends, etc. Endo residency will require your full attention/commitment once it starts. Hope that helps, feel free to DM me if you have any other questions.