I know Keltia is from the same French Company as Yggdrasill and uses the same mechanics. I have several of the books and have run crossovers a few times.
Of the ones that are not listed as cross overs only Lone Wolf Adventure game is not Cubicle 7's. Lone Wolf was by John Denver (who passed away this year so I have no idea about the future of that line).
If I click on the books title it is not found but I can still download the PDF by clicking on Download.
Edited by lordmalachdrimAdditional Info