Publish collection of papers into a book? I have a very general question. Say I have written 10 papers, published them independently in different journals over the years - could I then just take these papers and make a book out of them and publish that, or is there then a copyright on the published papers which would prevent me from doing that?
It is very likely that you would have assigned to the journal publishers the copyright to some or all of the papers you published, so you would not be allowed to publish the book without getting permission from those publishers. With that said, book anthologies of previously-published scientific papers are fairly common, and my understanding is that journal publishers are generally inclined to give their permission for a paper they hold the copyright to to be published in such an anthology.

A separate question is who would want to publish your book anthology. Unless you are a famous academic, I don't think such a book would be of interest to mainstream academic publishers, regardless of whether you need or can get the necessary permissions.