The following materials are available in PDF format for free. Consider downloading a copy of each so that you'll have them to reference for meetings and in between game sessions.
Labyrinth Lord Revised rulebook: (no art)
Advanced Edition Companion rulebook: (no art)
Mutant Future: (a post-apocalyptic role-playing game 100% compatable with Labyrinth Lord)
A Quick Primer for Old School Gaming: in case it's been a while since you played an old school role-playing game. Especially helpful if you've never played early D&D at all.