There is no doubt that Kevin's favorite book has to do with the legend of King Arthur. This, of course, is the legendary story of King Arthur and his Knights of the Round Table, with Queen Guenevere being another important character.
The reader knows this is Kevin's favorite story for a couple of reasons. First, Kevin has given a fun nickname to his mother as a result. Kevin's mother, whose name is simply "Gwen," is given the title "Fair Guenevere." Max, in a very cute exchange, asks why Kevin calls his mother "Fair Gwen of Air." This gives Kevin the opportunity to tell Max the story.
There is another important reason why Kevin really likes the story of King Arthur: it is a story of a "wimpy little kid" just like Kevin who did an amazing thing by pulling the "magic sword stuck in a big stone." No one else could do it except Arthur. Just like Kevin, Arthur had hopes of doing great things for his kingdom.
A final reason why Kevin likes the King Arthur legend is because of the armor the knights wear. Kevin says that, because of their armor, the knights are "like the first human version of robots." Kevin can relate to this because of the braces he has to wear on his legs.