The PRS has a built-in 802.11g radio, so obviously data was already being transmitted over ethernet. It also has a web browser, so again it obviously supported HTTP. Normally the browser only supports text/html though, which precluded a lot of stuff I actually wanted it for.
PRS+ helped fix this. It implements a lot of the things I thought were missing from the stock firmware; specifically non-html MIME type support. Actually it integrates this with the stock browser, which even made a user friendly interface possible. Authentication and encrypted HTTP unfortunately aren't implemented, which could make using it outside of home a bit impractical, but downloading from sites like project gutenberg normally doesn't need either.
Finally, to make everything I wanted to be able to read available via HTTP, I setup an apache webserver with mod_autoindex. The mod_autoindex module isn't completely necessary, but it gives nice HTML listings for directories which makes things a bit more user friendly.
A big thank you to the PRS+ developers, otherwise I would have been stuck. For anyone curious, it adds a lot over the stock firmware (file browser, screenshots, etc...).