You might not be aware of it, but all the Hilda books are available in print as well as in e-book format. The exception at this moment still is Hilda – Aiaia, book 9, as I am still awaiting the proof copy of that. (I refuse to make a book in print available unless I know it looks good.)
If all goes well, you should find all the paper versions of Hilda’s adventures by following this link. However, there seems to be a problem with this sometimes, so if the link does not work, please click your way over to http://www.lulu.com and search for ‘paul kater hilda’ (which is what the first link should do, but technology lately is more like magic than science).