One thing I am discovering about the difference between writing fiction and writing non-fiction technology books is that technology books are never really done.
With fiction, at some point you have told the story you are intending to tell. You may well have sequels to write, but a given story eventually is complete.
When you are working on non-fiction, there is always something more to add. Right as you are finishing the final review process, an important change is happening in the project core. You find a new tool that significantly improves on the painful process you documented in your book. The next version of the software is going to arrive after your book goes to the printer.
It never ends.