I am planning to write a book about an open source software product. Should I contact the creator of that software and get their explicit permission for writing the book? Or can I continue without consulting the creator and risk copyright infringement issues?
Copyright covers the expression of an idea. The author of the software has copyright on their code. But the result of that code is a subject in the public domain that anyone is free to write about.
On the other hand, you should certainly write to the author of the software and tell them you are writing a book about it and ask for their cooperation. For one thing, this will impress a potential publisher. For another, if you can get an endorsement from the author of the software or maybe get them to write an introduction for the book, that will really help to market the book (which in turn will impress the publisher.)