The International Society of Guatemala Collectors
https://guatemalastamps.com/ have recently released their impressive catalogue of that country's stamps & postal stationery and generously made it available for free download on their site.
Very comprehensive, with detailed information on printing, plating, varieties, forgeries, etc from the beginning to 2021. At 1073 pages in colour it is a very large PDF (~800 MB), so fair warning on that.
~Greg