Style Guides
Chicago Manual of Style: Subscription is $43/year (free with membership to Editors Canada).
APA Style: Print edition $30-60.
New Yorker (just kidding, that’s not available to the public).
Elements of Indigenous Style: $12-20 print / ebook.
Check out Owl at Perdue for a great academic guide to citations.
Dictionaries
Merriam-Webster (American): free, but there is a membership version for $50/year that gets you expanded definitions, etymologies, usage notes, no ads, and better browsing capabilities. Good for capital-N Word Nerds.
Oxford English Dictionary (British): 100 GBP/year. No free version, but you can get it for free with many library cards.
Canadian Oxford Dictionary 2nd Ed.: ~$50 print (no online version I can find yet).
Australian Oxford Dictionary 2nd Ed.: ~$30 print.
UrbanDictionary.com (So you can understand The Youth).
Resources
The Copyeditor’s Handbook by Amy Einsohn and Marilyn Schwartz, accompanied by The Copyeditor’s Workbook.
The Subversive Copyeditor by Carol Fisher Saller