Captain Salt in Oz, from 1936, was the first Oz book without color plates (not counting Road to Oz, which didn't have color plates but did feature pages of different colors of paper). This is also the only Oz story that never quite gets around to arriving in Oz itself. The characters are all Thompson's own, with no Scarecrow, Tinman or other traditional Oz celebrities - but as the main characters were already introduced in Pirates in Oz, the book fits right into the series.
I've always liked the way John R. Neill used the ship's wheel to create the word Oz for the cover title - this is the only time he did anything of that sort.
Handy Mandy, from 1937, is one of the odder characters Thompson came up with. A girl with seven hands takes a bit of getting used to - but I've always liked this book. Thompson's writing can be fairly slapdash at times, but the overall breeziness is enjoyable.
The spine of Handy Mandy in Oz used a stamped image of the main character's head. This same head was used the following year on Silver Princess in Oz. I also have a circa 1939 copy of Dorothy and the Wizard in Oz with the same head. Handy Mandy got around!