By the time Hidden Valley of Oz was published in 1951, the Oz books had been around for 50 years. This title introduced both a new Oz author, Rachel Cosgrove, and a new illustrator, Dirk Gringhuis.
I can't say I'm fond of Dirk's illustrations - for me, they lack the fanciful charm that was always present in the work of John R. Neill, even on his off days. They do give the book a very contemporary 1950's feeling, and this was the start of a time period when Reilly & Lee made several attempts to update the Oz books, with new covers and new illustrations. Eventually, they reverted to the originals.
For many years this was the only Oz book published by Rachel Cosgrove, but in 1993 the International Wizard of Oz Club published her second Oz title, The Wicked Witch of Oz. This was illustrated by Eric Shanower. Hungry Tiger Press has also published several short Oz pieces by Cosgrove.