One interesting item is a collection of sticker books, or Albums Autocollants. They are for sale from Garneau, and I saw the first one below on Ebay.
This says 1979, and ended up selling for the low asking price of $4.00. I guess French stickers are not all the rave. These others I've only seen on Garneau
Caroline a Bicyclette
Caroline en Vacances
Youpi a la Mer
Bobi et Eglantine
Youpi et Caroline
Apparently there are also some novels of the Caroline series as well, only in French. This includes some by Probst himself (or at least the drawings are by Pierre, see the Lelio piece below) as well as a few sanctioned knockoffs, with his characters but by someone else. One of them says © 1979, and lists the following as other titles:
Caroline Pouf et Youpi
Caroline Chez M. Belazur
Caroline et la Poupee Mecanique
Une Recompense pour Caroline
Caroline et le Perroquet
Caroline et le Serpent a Plumes
Caroline et le Train-Surprise
Caroline et le Ballon Voyageur
Caroline et la Pepite d'Or
Caroline et la Petite Sirene
Here also is a picture of a phonograph of a Caroline story, perhaps read by Pierre?
Also at some point in Pierre's career he did some artwork for some more educational books with titles like Nature Around the Year and others.
Finally, I recently saw this for the first time. I don't know how it ties in to other books at all, but it is supposedly by Pierre as well.