Thuvia, Maid of Mars

The fourth book in the series and the first that doesn't feature John Carter as the protagonist; instead his son Carthoris is dashing around Mars trying to rescue the beautiful Thuvia and avoid being framed for her kidnapping. There's certainly no deep plot here, all action and adventure, and Carthoris is practically indistinguishable from his father as a protagonist. The ending is also rather abrupt and could have used some fleshing out.

I've always rather liked this book though, as Thuvia is the first female character in the series to actually have any degree of depth. She determines her own fate and saves herself/companions more than once, making it a refreshing change from Dejah Thoris just standing around looking beautiful and waiting for John Carter to rescue her.