If you don't know where to start, you might find it helpful to look at the books we have in the stacks. These are the Library of Congress sections paired with their corresponding subjects, to help you browse for books:
QH 1-278.5: Natural history
QH 301-705.5: Biology, including:
QH 359-425: Evolution
QH 426-470: Genetics
QH 471-489: Reproduction
QH 540-549.5: Ecology
QH 573-671: Cytology
QK 1-989: Botany
QL 1-991: Zoology, including:
QL 360-599.82: Invertebrates
QL 614-639.8: Fishes
QL 640-669.3: Reptiles & amphibians
QL 671-699: Birds
QL 700-739.8: Mammals
QL 801-950.9: Anatomy
QM 1-695: Human anatomy
QP 1-981: Physiology
QR 1-502: Microbiology