Most of the books listed as part of the main series are available in the english langiage. All of them are available in german. The publication dates vary from 1972 and 1992 for the english releases, so they might be a little difficult (read: impossible) to find in your local main-stream book-stores.
Here are some helpful (I hope) tips on how to acquire these books, or, at least, how I managed to get a hold of them.
New: (Nov.01/07)
The whole series is currently being published electronically by Mushroom eBooks.
It is a great oportunity to get the books!
Surprisingly, the early volumes are relatively easy to obtain, although some digging is still necessary. And the prices are reasonable. I got mine by ordering them on-line from Abebooks. But other sources will also work. Among those are:
Also, some of the early books can be found e-published at Mushroom eBooks. They seem to have managed to obtain permission from the Bulmer estate to e-publish the first 12 books. Hopes are high that the rest of the series will also appear, but that remains to be seen.
New The first 3 cycles are available in hardcover and/or papercover from Bladud Books
Same as above.
Although the prices are significantly higher.
These volumes were never hard-published in english, as far as I know. However, they were e-published by Savanti Press. Unfortunately, they seem to be out of business at the moment and do not offer the volumes. Best way to get a hold of the files is through people, and here the Kregen@Y!Groups forum comes in handy. Become a member, ask around, and you will get them. This is how I got mine. :)
These volumes are not available in english, to the best of my knowledge. Pity, but true. Hopefully they will be published (or e-published) in the future, but for now only those of us who read german can enjoy the books.
I have not found a good source for the german editions yet, but truth be told, I have not searched very hard. I am still in the middle of re-reading the english books. :)
I think that the best way to find out about the books is to ask on the abovementioned forum Kregen@Y!Groups and hope that somebody will give you a good source.
The only other work on Kregen by Ken that I know of, the Wizard of Scorpio, was included in the "DAW Science Fiction Reader" (DAW#200) edited by Donald A. Wollheim and published in July, 1976. It can be found and ordered on the same on-line sources as given for the early volumes.
Most of the sources for Kregen stuff by other writes than by Ken I gave on the Fan Fiction page. If that is not enough, look on the forums, talk to people, and you will find what you are looking for. One good page that comes to mind is this one:
Also, please check my Downloads pages.