In the example that you give, an encyclopedia, I would answer yes, a mind map for each chapter and perhaps a more general simple one for entire work (i.e. simply the main point of the book and title in the middle and the chapters as branches).
An encyclopedia is really a collection of articles and any book that is similarly structured, in general should be treated during the entire PR process from prepare to activate as reading seperate articles.
If one is reading the great work with the purpose, "I want to find something in this book that will be of value to me for the purpose of x", then the PR is to find which article(s) one should spend more time on.
In other words, part of the answer to your question in in your purpose. One could PR 10 books (or articles or whatever), to find what one should spend more or less time on.
I think this is one the strengths of the PR system: to change the way we think of reading material to "weed out" or focus on, as needed.
Therefore, if one wanted to read sucha great work, what one mind maps would depend on the purpose.
If the reference work was indeed something we needed for a class, or a job or some other specific task or purpose, still one should treat each article as a seperate work and keep in mind and focused on one's purpose. This can help in cutting the time down and work spent. Because thought there may be a lot of information in the collection or even in a specific article, it is likely that we don't need all of it. And what we DO need, our other than conscious mind will help us with if we let it and direct it to.
Hope that helps.