Amanda and Maniac both like the 'A' section of the encyclopedia. Why do they both like it in Maniac Magee?
The "A" volume of the encyclopedia is the only one Amanda owns. She had gotten it at the supermarket, and then the supermarket offer had expired; there were no other volumes to be had. Amanda and Maniac are both of an age where they are interested in more challenging books. Reading the encyclopedia is a challenge, and so both of them enjoy "tackling" the encyclopedia, and the only volume available to them is "A."
Amanda owns a variety of books, including one about the Childrens' Crusade, and Lyle, Lyle, Crocodile. She is a voracious reader, and currently is reading the "A" volume of the encyclopedia from cover to cover, "from Aardvark to Aztec." It takes a long time to read an encyclopedia, and Amanda has "vowed" not to give up the book until she has finished it. Maniac is interested in the encyclopedia too, but does not get a chance to read it because Amanda always has it. He does find a way to read it despite Amanda's monopoly of the book, however, by getting up very, very early in the morning, finding where she has hidden it, and reading it awhile by flashlight.
In addition to the fact that the two children have a shared interest in the content of the encyclopedia, Maniac is probably even more motivated to read the "A" volume simply because Amanda is so enthralled by it. "The more Amanda would not let go of the A, the more Maniac wanted it;" when someone really is into something, it makes that thing seem especially appealing, so Amanda's extreme absorption in the book intensifies Maniac's desire to read it too even more (Chapter 15).