He means this Dragon's Egg:

https://en.wikipedia.org/wiki/Dragon%27s_Egg

The science in this book is as factual as hard science fiction can get. Zero handwaving or fantasy physics.

Bob Forward was not just an author, he was a career physicist.

>Robert Lull Forward (August 15, 1932 – September 21, 2002) was an American physicist and science fiction writer. His literary work was noted for its scientific credibility and use of ideas developed from his career as an aerospace engineer. He also made important contributions to gravitational wave detection research.

There aren't many people who can do both those things well.