How turn a document into a book?

re: ... how do I turn a long document (150 pages) into a book?

It's trivial if the long document is the only "chapter".

1. Create a new .book
2. Add the document .fm file to it
3. Generate the TOC & IX files as desired.
4. Create and add any front and back matter.
5. Save and close all component files. Save the book.

The original document file continues to exist.

Now, if you plan to split the original .fm into separate chapter files, existing cross-references might pose a challenge.