I’m in various stages of reading all of these.
Foundations of Astronomy by Back and Seeds, 2010. Make sure you get the International edition, since you do not want to wrangle miles and pounds and other non-metric nonsense. Very accessible basic and less basic information about everything in space.
Titan Unveiled by Ralph Lorenz and Jacqueline Mitton, 2010 edition. All the latest about Titan. Written in non-scientific style.
Unmasking Europa by Richard Greenberg 2008. Latest information on Jupiter’s moon Europa. More scientific in style than previous title.
The Starflight Handbook by Mallove and Matloff 1989. This was recommended to me by the good people of the Analog forum. Although published a good while ago, there has been little change in the basic concepts and ideas. It’s making the ideas work that’s the hard part. If your setting includes anywhere near realistic space travel, I’d put this on your must-read list. Even if you decide to hell with the laws of physics, you know what principles you are breaking, and you know where to insert your magic device.
I bought all these books through abebooks.com