I recently finished my PhD thesis which will be published as a book soon. Now I'm wondering if it's possible or allowed to submit parts to a journal?
Normally the process would be the other way round I think: Submitting papers and "gluing" them together for the thesis. I've read the guidelines of some journals which state "that the work described has not been published before" or that "Papers must present scientific results that are essentially new".
In general, this is allowed, even encouraged.
The answer depends on what kind of book you are publishing. If it is the regular dissertation, then you can publish in journals. If it is a properly published book by Springer or equivalent, then I doubt that you can publish again.