A couple of pretty good references (with a 15 year old Boxster, you should probably consider getting one or both):
A few online sources I've used:
DIY Projects: Do-it-Yourself Repair and Maintenance for your Porsche
Pelican Parts - Porsche Boxster 986 / 987 and Carrera 996 / 987 Technical Articles
There will be a huge overlap between the pelican parts website DIY and the 101 Projects book---Wayne is responsible for both. The website has some additional photos, etc. The book is handy since it's easy to carry out to the garage for quick reference while you work. You can also get a LOT of info by just Searching this and other Boxster sites.