The are hundreds of incredible references online dealing with the history of computing, and especially about the TI-99/4 and TI-99/4A computers. I list as many as I can find for your reference:
- Centre for Computing History = This is a UK-based, general computing history website that has numerous articles involving the TI-99/4A.
- Eli's Software Encyclopedia = This software focused website includes many articles about TI-99/4A software. It's written using mediawiki (the same software as this website and Wikipedia).
- Ninerpedia = A great resource run on Mediawiki (the same software as wikipedia and this website).