FORTRESS is the only solo book I've written when I didn't want to write it.
When Jim Baen left Tor Books to found his own publishing house, Baen Books, I intended to continue working for both men. (They're both friends of mine, and friends to one another.) To emphasize this, I told Tom to decide what he wanted me to write as my next book. He wanted a sequel to Skyripper (which has its own entry) that involved using the Strategic Defense Initiative (the Star Wars defense) against invading aliens.
Boy! did I not want to write that book.
Partly that's because I had doubts about SDI technology. (I still have doubts, but considered as a political initiative SDI was a brilliant stroke that put paid to the Soviet Union.) I could around that, however; the real problem was that I didn't want to go back into the mind of Tom Kelly.
All of my characters are either parts of me or other people as I filter them through the medium of my own mind. Tom Kelly is me on a very bad day.
Fortress and Skyripper have many virtues, including great car chases, gunfights, and an examination of class structures and intraservice hostility within the US intelligence community (which last went right over the heads of some British reviewers, I found to my amusement; Brits are unused to thinking of class outside their own national terms). Kelly also has virtues; but he won't compromise, and he's absolutely ruthless.
Like I said, me on a bad day. I was glad to finish the book and be able to bury that character a little deeper in my psyche again.