This is collection of FAQ sites that have bucket loads of information for the TI-99 line of computers.

arcadeshopper.com maintains an all encompassing FAQ for the TI-99. If you have a question this is a good place to start: https://www.arcadeshopper.com/wp/ti-99-4a-faq/
nouspikel maintains ‘the’ technical resource for the TI-99. This is the place to go for all things technical: http://www.nouspikel.com/ti99/titechpages.htm
ninerpedia maintains a good TI99 Wikipedia on various subjects of interest and also includes information on the TI 99/8 and the Geneve: https://www.ninerpedia.org/wiki/Main_Page
TI-99/4A-pedia has a nice list of TI cartridges and also of programmers who wrote the software for the TI-99: https://ti.mysweetweb.com/index.php?title=Main_Page
Atariage TI99 Forum of hardware & software resources. Lots of links to various resources around the internet. You need an Atariage account to download: https://atariage.com/forums/topic/267055-new-to-the-group-ti-994a-faq-hardware-and-software-resources-read-first/