A well known publisher here, Bentley, is bringing out a Boxster maintenance manual set shortly, some time this month. It is expected be US$100 and is said to be excellent. Sign up here to be notified when it is available
http://www.bentleypublishers.com/product.htm?code=pb04
There are also Porsche work shop manuals you can buy online for US$20 for download, it is a 600mb download in PDF format but isn't great. They are ok but hard to follow and don't go in to detail. Worth $20 maybe but not much more.
Having said that they seem to have disappeared from the web site, they were on here
http://www.workshopmanuals.co.uk/workshop_manuals/cars/index.html but people have said they were unofficial, maybe Porsche shut them off. The ones on ebay are the same as these and in my opinion are not worth the money.
I have no affiliation with any of these, just passing on the links and what I have heard.