I, too, read a dumpload of literature and would love to have a good dictionary and encyclopedia to work with. The only ones I found for the ppc are freaking huge and expensive, $50 for the dictionary and $50 for the encyclopedia. However, they are the most thorough of any program in either category.
World Book Encyclopedia
World Book Dictionary
Look for them at Handango. If you buy them, you'll need a memory card to store them, because they're disgustingly huge, like 70 megs per program, I think.