I've looked previously (here and here) at a few of the Dutch reprints of Billy's Boots that have been produced. That was quite a lengthy series of reprints volumes - stretching to at least 33 volumes. I've got most of those volumes (I'm just missing volumes 23 & 25) but there are a few more things that you could be looking out for once you've collected all those Billy's Boots volumes. This is by no means a complete guide to what's out there, it's just a introduction...
This is a (nearly) A5 sized text novel, approx. 70 pages long. Only one internal illustration.