The Johnny Maxwell books were clearly written with younger readers in mind, and unlike some of Pratchett's other books for younger readers, there aren't that many inside jokes for adults. In general, the books go a little lighter on the trademarked Pratchett humor than most of the others. As usual, Pratchett weaves interesting if not overly complicated stories, but the attempt at making believable, real world characters fell a little flat in my eyes. Maybe I'm too old. Still, it's Pratchett so you know it's going to be a decent read, and if you're, say, 12 to 15 years old, even better.
About spelling problems, there are no more errors than in most ebooks. The other reviewer seems to be referring to a segment early in the first book where Johnny, who is 12, is typing and writing things like can@t.