I downloaded what Gutenberg had and somehow started with this one as I could not find the correct order for the works.

I have six more by LeBlanc. I am not sure at the moment if they are all Lupin stories or not.

I did not put them in the collected forms that Harry loves so well as the information I found said that there were twenty Lupin books in total.
There should be Leblanc fan sites available. If you find out about the correct order then please email it to bookshelf at pglaf dot org (which gets forwarded to me) and i will update the bookshelf page at Gutenberg.
Some of the Lupin books are probably still in copyright.
I find the "anthology/omnibus" (is there a difference? ) format useful for a series like, say, the "Raffles" or "Father Brown" stuff because it means that it's all together in one place on the Reader that way.

Obviously, though, if you're not sure about the order of the books, or whether there are any "gaps" in what you have, doing individual conversions is a lot better.
The Fantastic Fiction site is excellent for finding the chronological order of an author's work.

You can find all the works by Maurice Le Blanc here:

http://www.fantasticfiction.co.uk/l/maurice-le-blanc/
Yes, I agree. "Fantastic Fiction" is where I get all my "series" information from. Handy for all sorts of things - the order of books, publication dates, etc.
Sorted the LeBlanc entries in PG bookshelf.