Old Twentieth is set in the future, when much of humanity has perished in a gruesome cataclysmic struggle. A treatment called BCP has been discovered - it confers near immortality on humans. Unfortunately, it is hideously expensive. When word breaks out that the wealthy of the world are secretly looking forward to greatly enhanced lifespans, violence and anarchy ensue. A deadly chemical called Lot 92 wipes out billions. In the end, Earth has been greatly depopulated, home to a relative few humans who are nearly immortal.
The narrative of Old Twentieth begins aboard a set of five ships that are enroute to Beta Hydrii. It has an Earth-like planet, possibly it could support a colony. Not that the depopulated Earth NEEDS to find room to expand, but when people have such long lifespans, this is the kind of project that becomes possible. One of the ways the immortal humans entertain themselves is with a virtual reality machine. The protagonist of the story, Jake Brewer, is an engineer who works on the machine. The colonists immerse themselves into past realities. Because the twentieth century has the most amount of historical data, that is the reality that the machine simulates (this the source of the title in the book - the VR machine takes the people back to the old twentieth century). Not all the trips are happy and fun - a lot of the virtual reality trips involve world wars or other catastrophes - but if you are an immortal, this is thrill seeking at its finest. Any one in the simulation can exit at any time if the situation becomes too intense.
Haldeman sets this up wonderfully. I loved the fleet of five ships, outward bound on a journey that will cross light years. I wish Jake Brewer had spent a lot more time wandering around, giving us a tour of the place. But Jake is pretty busy, because someone has died while experiencing the virtual reality machine. Is this a coincidence? Has something gone wrong? Jake tries to decipher the problem, and notices that not all the simulations are perfect. A VR construct gives Jake a message - "We have to talk" Clearly, this is a message from the virtual reality program - which essentially is an artifical intelligence. It is well done, it reminded me of HAL 9000 machine.
The ending seemed abrupt to me. It ended the way I thought the Matrix movies were going to end. This is a good science fiction book.