If one looks at the Baen Free Library (Baen Books), they have an interesting attitude toward e-books.
They sell e-books of their stuff - even serialize some books in progress. But, if certain books form a series, after a while they put the first 1-2 books in the series in the Free Library, knowing that if you REALLY like the series, you'll not only buy a dead tree version of the books you downloaded, but also by the later books in paper or elctronic form - maybe even both.
The alternative is, after all, that the casual reader will read the book from a copy in a local library, or buy the book and resell it if they don't like it - both of which will have worse financial repercussions to the author and publisher than not having the series' intro books free on the site after they've been paperbacks for a year.
For RPGs, this would best translate as putting the core rulesets (not the whole books, just the rules engine) free (example - the GURPS Lite ruleset), and dividing the non-core books up into two categories-
1. "continuing relevance" items = those that would have a long-term market in paper form, as copy-protected, pay downloads,
2. "timed obselescence" products = ones that will be eclipsed/subsumed into later products, or never be reprinted after the initial paper print runs, that will be pay products originally, but changed over to discounted or free downloads as the supply of printed copies disappears from stock.