Books are shelved in general subject categories using the Library of Congress classification system. You may want to supplement your use of the catalog with browsing shelf areas for your topic. Below is a brief listing of some of the subject locations in the field of biology. These are all located on the main floor of the library.
Q -- Science - General
QD -- Chemistry
QH -- Natural History
QK -- Botany
QL -- Zoology
QM -- Human Anatomy
QP -- Physiology
QR -- Microbiology
R -- Medicine
S -- Agriculture
The library doesn't routinely add textbooks to our collection (they cost too much and go out of date so quickly!) but you can find some general biology books, including older textbooks, by searching our catalog and browsing the QH section of shelves. There are also reference works across the aisle in the Reference - QH section that will have definitions. There is also a basic Principles of Biology textbook on the course reserve shelves.
Also, these free open access textbooks are available online.
And these books are available through the EBSCO ebook library, though may be electronically "checked out" - only one person at a time can use it.
These books can't be checked out, but they might have useful background and factual information. Though they are written like Wikipedia articles, the authors are experts in the field and they sometimes provide a scientific focus you won't find at Wikipedia.
Bioethics by 
Hormones, Brain and Behavior by 
Encyclopedia of Biodiversity by 
Encyclopedia of Animal Behavior by 
Encyclopedia of Microbiology by 
Encyclopedia of Entomology by 
Brenner's Encyclopedia of Genetics by 
