Petzl has always been great about publishing free how-to's and other educational information for climbers. They tend to have just the essential information, conveyed in very easy to digest graphic format. One of their latest, Belaying in a Climbing Gym, is a great resource for climbers new to the topic.
BIACG is a PDF format e-book you download straight from Petzl's website. Eleven pages cover seven topics: tying in with a figure 8 knot, partner check, belaying a climber on top rope, belaying a climber on lead, lowering, clipping and errors to avoid.
It's a great review not just for beginners but for anyone who hasn't climbed in a while, climbs seasonally or just needs a periodic refresher. We all have friends who want to learn to climb, and often their first stop is in a gym. Send them this link and they'll be well prepared for their first day to earn their belay card, and they'll have an easy-to-read reminder when they need it.
The book is part of the Tech Tip Guides series. There's a second one available as well, How to Prepare and Plan for a Ski Tour. Get 'em while they're free!