Spoiler level: medium
Frederik Pohl's Heechee trilogy never fails to produce surprises for me. I absolutely loved the first book, Gateway, but I figured that was all there was to tell for the book's protagonist, Robinette Broadhead, and that further Heechee books would focus on different characters, similar to Asimov's Foundation trilogy. So when I heard that the follow-up book Beyond the Blue Event Horizon still focused on him I was wondering how that would work. The answer was, brilliantly.
Heechee Rendezvous ties up the plot threads opened up in Horizon, giving the Heechee series a good place for closure while at the same time opening up the door for even wider stories. (And a quick wiki search has shown me that there are two more novels and a collection of short stories left! Oh boy! I'm not done after all!)
There are many things that make this book special, my favorite of which is seeing how Robin has grown as a person from the first book. His demons have been mostly exorcised, and he's a much more compassionate (if not still practical) man. His own survival has become one where his every need has been met for years, so his goals now are to the survival and growth of mankind. And Wan, the man-child discovered in the previous book, is his exact opposite: the universe revolves around him, and having all his needs met has only reinforced his selfishness. We see both extremes of humanity in these two characters.
The book also digs deeper into artificial reality than ever before. AI characters have always been a part of the Heechee books, with Robin's counterpart being Sigfried von Shrink in Gateway, followed by Robin's virtual Albert Einstein in Horizon, but they were always treated as simple programs imitating intelligence; here the line is firmly crossed into sentience. And the question of why the Heechee disappeared in the first place is finally answered, with major implications for the future of the human race.
It's a shame that most of the books are out of print. Finding them is no trouble thanks to the internet (half.com has most of them for $0.75), but I would think that there was more demand for these books since Gateway is considered a classic. I found Heechee Rendezvous at a used bookstore over a year ago and bought it even though I had yet to find Beyond the Blue Event Horizon, and I'm glad I did. It was definitely worth it!