Most large libraries carry books on guitar repair, and I suspect you'd find Nashville is better than most on the subject - loads of people must ask for them! Authors to look for are Teeter, Irving V Sloane and Hideo Kamimoto. If all that reading gets you down, there's a very good repairer called Obie Jones, who should still be in the book. If the guitar is not particularly valuable, filling the gap with a plaster-based filler coloured with tea would cover the damage and still allow for a more professional repair at a later date, as it will easily scrape out.