I've seen a few links here to fab new ways of making squillions online with public domain works and I thought i'd add my pennyworth of research.
There are some products here claiming to give away all the 'secrets' of making money with public domain publications and to be honest it isn't rocket science.
Essentially anything published prior to 1923 is classified as public domain and ripe for reproduction-unless new copyrights have been legally created by the original publishers.
As the net is awash with some great library sources like the Guttenberg press and the Duke Library etc etc, who publish painstakingly scanned works in to the digital domain it is perhaps tempting to just nick them / reproduce and sell -however ethically and personally speaking I believe I should atleast possess an original copy myself to scan and reproduce,either in un-abridged form or whatever.
Using someone elses often time consuming scans is not exactly illegal [ if its material pre 1923],but it ain't cool either- so if you are thinking of getting in to this market,get the original copy.
There's tons of stuff out there on ebay folks !
and its definitely an interesting new market if it is done tastefully.
Anyway I'll finish this post with the following clarifications for those of you interested:
Pre 1923 -
public domain -period
28 years copyright protected initially
can be renewed for up to 67 years- if it hasnt been-then it is in the public domain
28 years initial copyright -now automatic extension for 67 years