The most widely used style guide in book publishing, covering grammar, punctuation, citation, and formatting standards.
The Chicago Manual of Style is a comprehensive reference guide for writers, editors, and publishers that establishes conventions for grammar, punctuation, citation, and manuscript formatting. First published in 1906, it has become the default style authority in book publishing, used by most traditional publishers and many indie authors. If your editor marks up your manuscript with questions about comma usage or citation formatting, they're almost certainly referencing Chicago.
Consistency makes your writing look professional, and Chicago is the standard most book publishers expect. Knowing the basics saves you from embarrassing inconsistencies and cuts down on copy-editing time. You don't need to memorize the whole thing, but understanding how to use it as a reference will make your revision process smoother.
Pick one chapter from your manuscript and check it against three Chicago Manual rules: serial comma usage, dialogue punctuation, and number styling (spell out one through one hundred). Mark every inconsistency you find and correct them. Notice how many you missed on previous read-throughs.