Forgotten Futures~ pdfs
I’ve been a supporter of Forgotten Futures for years, long before the rise of steampunk the game’s author, Marcus Rowland, was creating excellent game material from vintage sources all of them FOR FREE. He continues to do so.
Additionally he has been adding new (free) historical items to his website, check them out here: http://www.forgottenfutures.com/
Several of the books for Forgotten Futures are now available as pdfs, you may obtain them (for now charge) from DriveThruRPG: http://rpg.drivethrustuff.com/browse.php?manufacturers_id=2571
I could not recommend this game more highly!