I've written four books, and several years ago I developed "Cockcroft's law of book writing". This states that a book will grow in size as you write it, and that the number of pages left to write will increase as you write. This seems counter-intuitive, but it has been confirmed many times in practice. I hope this posting provides some useful advice for writers, and helps people finish what they have started.
To make a concrete example, let's say you decide to write a book and you come up with an outline that adds up to 200 pages. You start work and write 50 pages, then, when you revisit your outline to update the page count estimates, you find that they now add up to 300 pages. You wrote more than you expected to cover each subject, and discovered more subjects that needed to be discussed. The essential problem here is that there are now 300-50 = 250 pages left to go. Before you started you only had 200 pages left to go.
This problem is recursive, if you write another 50 pages you will find that you have now written the first 100 pages of a 400 page book, and you now have 300 pages left to write. This explains why there are so many people who have written part of a book, but never finished it.
The aproach I took in writing my later books was to maintain a spreadsheet that tracks the pages left to write or edit, update it very regularly, and generate a plot with a trend line from the data. You can then see when (or if) you will finish the book. In order to get the trend line to target a specific delivery date, you have to force the number of pages left to go down. You do this by writing pages that you promise never to edit again, and by deleting whole sections and chapters. I deleted three entire chapters from one of my books to get it finished.
Another problem you can run into is that the content you wrote at the start of the process is less well written than later content, so you think you have finished, re-read parts of the book that were finished ages ago, and discover that it needs a complete rewrite.
I often get asked if I will update my Sun Performance and Tuning book, and I don't intend to do a third edition. This is mainly because I'm interested in other things, and I'm no longer up to date with the subjects I would need to cover. I have sketched out a possible book on capacity planning with free tools, and the trend line on that book is nice and flat. I haven't really started writing it, and so it hasn't started getting bigger yet....
My good friends Jim Mauro and Richard McDougall are closing in on the end point for Solaris Internals 2nd Edition. I've been looking forward to it for a while, and its going to be a monster book, covering how Solaris 10 really works, lots of DTrace based examples, and is going to be the essential companion for anyone looking at Open Solaris.