Owner's Manual (under Online Docs Menu)
Radio Manual (Becker site charges via paypal)
or here (for free, you may have to register) (under Quick Menu, Porsche Owners Manuals (they keep moving the menus around, you may have to hunt a bit))
ODBII Codes (under Online Docs Menu if direct link doesn't work)
Technical Service Bulletins (list, you get the text of the TSB's only after donating to RennTech)
Get a parts list (called the "PET") by going to https://techinfo.porsche.com/ and select "Country-USA", select "Workshop Information", Select "Genuine Parts Catalog", select "Catalog 20" "986 Boxster" and download. This will give you a 400 page .pdf file from which pages can be printed as needed.
Get the multi-thousand page Boxster Maintenance Manuals the dealer uses from here for free as PDFs but the downloads are erratic and it may take multiple trys per manual section. Not sure how up to date these are but they often help and there are wiring diagrams, diagnostic flow charts, references to special tools, etc.