Finding used travel books

After I griped about how expensive travel books are, Cornelius suggested that we still try to find a used book for each country we may visit during our travels (so that would be 13 in South America, 7 in Central America, and Mexico). So far we've mainly stuck with Lonely Planet, but we should consider branching out I think. Any suggestions for other good travel books to check out? Amazon and Powells didn't have many used Lonely Planets at much of a discounted price, but I found 9 (covering 10 countries) on Alibris - each for $6 including shipping and handling. Still looking for the books covering Nicaragua/El Salvador, Honduras, Colombia, and Guyana/Suriname/French Guyana. Actually, Lonely Planet doesn't cover these last three in a separate book, and I haven't found other guides that do - suggestions appreciated here as well.