A rollicking trip through time; Max and Me and the Time Machine, by Gery Greer and Bob Ruddick. New York: Harcourt Brace Jovanovich. 114 pp. $11.95. Ages 10-12.

''Max and Me and the Time Machine'' is one of those stories that's rather preposterous, totally unbelievable, and yet, still rather good reading. After Steve buys a time machine (at a garage sale) that can take him back to whenever he wants to be, he and his friend Max decide to try it out. Unable to resist the lure of being knights in armor, they choose the Middle Ages. Time travel not yet being a perfect art, the two boys find themselves in the Middle Ages all right, but with Steve inhabiting the body of Sir Robert Marshall (the Green Falcon), while Max ends up as his horse. They soon find their hands full, between jousting with the evil Sir Bevis Thorkell (the Hampshire Mauler), wooing the fair Lady Elizabeth, and avoiding the ministrations of the crafty Clarence Gathergoods (doctor, alchemist, and barber extraordinaire). Adding to their problems are the eccentricities of their time machine, which manages to change Max from one character to another.

''A Connecticut Yankee in King Arthur's Court'' this isn't. But for youngsters who enjoy imagining what it would be like to travel in time, it does provide some rather innocuous reading.