A style guide is a handbook that gives details about how to punctuate, capitalize, cite references, and do other details of writing.
Some of the library's style guides are online, and others are in General Reference on M-Level, or on D-Level in the Blue Labels, in the PE1408 section. They include:
See Citing Sources for more on the big four style guides.
In addition, see the Purdue OWL's site for tips on resources for subject specific writing.
Usage manuals are books that illustrate how to use English correctly.
Most of these are on D-Level in the Blue Labels, in the PE1460 section.
The best-known guide deals generally with the essentials of how to write "plain English"; this is the classic
-- Gen Ref on M-Level
-- D-Level Blue Labels
-- online through Bartleby.com (listed as "Strunk's Style" on the pulldown)
Several recent usage manuals are quite funny and use clear examples. These include: