ISBN 0064402223 - Not the best thing I've read in kids' books lately, but good enough to hook some of the reluctant readers, especially boys.
Steve and Max are best friends, which is a good thing, because otherwise Max might have killed Steve by now. Who could get away with convincing you to eat a dog treat, if not your best friend? So when Steve comes to the clubhouse with a $2.50 time machine, Max isn't surprised. He doesn't take it seriously, but he's not surprised at all. After some explaining, he agrees to go with Steve to the year 1250... even if the contraption doesn't look like it's going to go anywhere. When Steve thinks Max is getting cold feet, he flips the switch before Max knows what's going on and they find themselves in the bodies of Sir Robert, a medieval knight, and his horse!
Stunned that it worked, thrilled with their success, the boys learn how things work in the Middle Ages. From quack doctors with potions to romance with an Earl's daughter, they're enjoying themselves quite a bit. Now all they have to do is hope the time machine brings them back before one of them is killed!
This is the kind of book that could easily translate into a series, with the boys travelling through time. Since the inventor of their machine, Professor Flybender, went off in search of Atlantis, never to return, there's a story to be told there - and if they tell it, I'll read it!