Here's how it breaks down:
* 500: Natural Sciences and Mathematics
* 590: Zoology (the study of animals)
* 595: Invertebrates (animals without backbones)
* 595.4: Arachnids (a class of invertebrates that includes spiders, scorpions, ticks, and mites)
* 595.44: Spiders (the order Araneae within the class Arachnida)
So, books about spiders would be found in the 595.44 section of a library.