Additional GEOS Resources

Information

GEOS on Wikipedia: Not a bad starting place for general information.
Bo Zimmerman's GEOS site: General-interest GEOS site with several screenshots, some useful developer documentation, and Bo Zimmerman's personal GEOS programming projects.
Direct link to the Commodore GEOS FAQ file: Bo Zimmerman's Frequently Asked Questions (FAQ) file from the comp.sys.cbm era. A bit dated at this point, but there's some solid information here.
GEOS article at OS News: Detailed review of GEOS.
GEOS for the Commodore 64: ToastyTech's brief tour of GEOS with screenshots.
Exploring Commodore GEOS Desktop Options: Overview of alternatives to deskTop: GateWay, GeoShell, CLI, TopDesk, and DualTop.
Unofficial mirror of Cenbe's Lyon Labs GEOS pages: When it became obvious that the Lyon Labs site was at risk, digital preservationist Lillian Skinner found an opportunity to scrape the site. This mirror preserves most of the Commodore content but none of Cenbe's personal pages.

Bo Zimmerman's Commodore FTP: GEOS subdirectory: A large collection of GEOS applications, desk accessories, fonts, artwork, and documentation. Includes the obscure GEOS 3.5 port for the Plus/4.
Commodore Software's GEOS files for the C64: Includes Bruce Thomas's massive geoSpecific collection of public-domain GEOS software. Registration required for file downloads. The C128 section has some additional GEOS files for the Commodore 128.
GeoUTools: GEOS tools for the Commodore 64 Ultimate II+ cartridge: an auto-exec to set the system clock, bug fixes for GEOS 128, and additional configuration tools. The author, Xander Mol, has confirmed that these tools should work on the new C64U with the release firmware.

PageTable blog: Dozens of technical articles related to GEOS. I recommend Why do C64 GEOS Boot Disks Break? and the 9-part series Inside geoWrite.
GEOS source code: Reverse engineered by Maciej Witkowiak and Michael Steil.
GEOSLib cc65 library documentation: cc65 is a complete cross-development package for 65(C)02 systems, including a powerful macro assembler, a C compiler, linker, library, and several other tools. The GEOSLib library documentation may be useful for general GEOS programming.

GEOS 2.0 Manual: Archival scan of the original GEOS 2.0 manual from Berkeley Softworks. Worth your time; there's a lot of functionality that you're unlikely to figure out purely through trial and error.
Hitchhiker's Guide to GEOS v2022: The definitive source for GEOS programming. Last revised in 2022 by Paul B. Murdaugh, author of dualTop. Includes information sourced from Berkeley Softworks's original 1988 Hitchhiker's Guide along with the GEOS Programmer's Reference Guide by Alexander Donald Boyce and Bo Zimmerman's 1997 revisions.
GEOS Magazine Articles: Listing of hundreds of GEOS articles, reviews, and type-ins from magazines like Compute!'s Gazette, Run, Commodore World, and geoWorld. Each search result links to a scan of the original article on archive.org.