I have an enormous amount of textbooks on marine biology- a special emphasis on fish (my field of study) but I also have quite a few textbooks on parasitology, virology, bacteriology, biochem, molecular biology, and aquatic microbiology.

I think the more you understand outside of your immediate field, the better you get at it. In my free research time, I spend time trying to slowly learn environmental chemistry to better understand how further pollution will affect my seafaring friends…. So far, not very good outlook.