I preferred this to the Sobel book. More academic, but a lot more technical detail.

The Quest for Longitude: The Proceedings of the Longitude Symposium Harvard University, Cambridge, Massachusetts November 4-6, 1993 Hardcover – November, 1996 by William J. H. Andrewes http://www.amazon.com/The-Quest-Longitude-Proceedings-Massac...

I guess it doesn't matter, after all that time, that poor Maskelyne's name gets destroyed for the sake of a good story.