Choo lent me The Mastermind of Mars by Edgar Rice Burroughs (1928) about a million years ago. I put it at the bottom of my pile because it is the sixth book in Burroughs' Barsoom series (hint: Barsoom is what the Martians call Mars), and I had only read the first one. Eventually, though, the cover called out to me too strongly and I decided to just take the plunge. Lucky for me Burroughs completely changes direction in this book, which serves as a pretty much stand-alone entryway into the world of Barsoom.
Our hero, Ulysses Paxton (yes), is dying on a WWI battlefield in France. He has always loved reading Burroughs' books about John Carter and his adventures on Mars, and in his final thoughts he focuses on the stars and thinks about how much he wishes he could have seen this amazing world. Then, through the wonder of astral projection (or something), he wakes up in the compound of a scientist on the red planet. Yay!
The scientist, Ras Thavas, quickly teaches Paxton the Barsoomian language and gives him a new name: Vad Varo. Ras Thavas has perfected a method of transplanting the brain and soul of one person into the body of another, and is charging wealthy Barsoomians tons of money to trade into younger and better looking bodies. He can also bring the dead back to life and transplant limbs and organs. It's a pretty good system really.
Paxton plays along as his lab assistant until he meets Valla Dia, a beautiful and benevolent young woman who has had her body switched against her will with an old and ugly empress. Paxton falls in love with Valla Dia's mind, even though she is stuck inside a crappy body, and vows to reunite her with her proper form. This leads to all kinds of adventures, and Paxton teams up with a revived assassin, a giant ape that has half of a man's mind (one of Ras Thavas' many experiments), and a proud warrior whose body was stolen by an evil courtier to woo away the woman he loved. Confused yet? Top that all off with some pretty funny critiques of religion and the masses and you have a very fun science fiction book that shows Burroughs at the top of his form. An excellent read.