Casual reading, practical advice or hardcore history? Believe me, the choice makes a big difference in cover price and availability.
For hardcore historical surveys of particular arms, you want to check out the catalogs of certain publishers. Collector Grade is the front-runner, followed by Mobray Publishing, Wet Dog Publications and Schiffer Publishing (prepare for sticker shock!)
http://www.collectorgrade.com/
https://gunandswordcollector.com/
https://www.wetdogstore.com/
https://www.schifferbooks.com/
For practical information on various firearm topics, try Gun Digest Books, Stackpole Books (mostly older titles now), Krause, Wolfe, and especially Skyhorse Publishing
https://www.skyhorsepublishing.com/search-results/?keyword=guns
https://www.gundigeststore.com/
http://www.stackpolebooks.com/
http://bulletin.accurateshooter.com/2016/03/krause-and-gun-digest-books-at-midsouth/
https://www.wolfeoutdoorsports.com/brands/Wolfe-Publishing.html
If I was asked for a good place to start, it would be with the digital collection of Gun Digest. These are bulky in paper form, but having the entire run in PDF form gives you a selection of articles running the gamut of firearm interests. This guy on eBay is offering a pretty good deal on the set at present:
https://www.ebay.com/itm/Gun-Digest...ions-Digital-Collection-Shooting/391257312518
BTW, a good source for remaindered bargain titles is Edward Hamilton -- I whole-heartedly recommend that everyone here gets on their mailing list:
https://www.hamiltonbook.com/
A non-exhaustive list of specific authors I particularly like include Patrick Sweeney, Elmer Keith, Peter Capstick, Julian Hatcher, George Nonte, Jim Corbett, Frank de Haas, Wayne Van Zwoll, Jim Foral, Tom Turpin, Edward Ezell, Maxim Popenker, R.L. Wilson, Bruce Canfield and Peter Kokalis.
Now out of print but still commonly encountered, I also quite like the Firearms Classics Library editions of older titles. Here are some shots from my personal library -- gun books occupy about 40 feet of shelving around my house.