Heechee Rendezvous by Frederik Pohl
This is the third book in the Gateway / Heechee series.
In this book we find a Heechee coming outside the galactic core's black hole to take a look at how things are progressing. He discovers humans have advanced more than expected. And one human has taken a Heechee ship and is going around peeking inside black holes around the galaxy. This is problematic for the Heechee - not only do the Heechee have some things hidden in black holes they don't want humans to find before they are ready, but there is also something in one black hole that the Heechee fear. They don't want humans opening Pandora's Box.
We also find Robinette Broadhead (from Gateway) is now an older man and one of the world's richest and most influential people. He and his wife are trying to help humanity by getting people to settle other planets, providing food using Heechee technology, copying the consciousnesses of dying people into computer storage, trying to reduce the threat from terrorists using emotion transmitting devices, etc.
The book fills in some gaps about the Heechee, it tacks on a little more onto the end of the Heechee saga, it talks about uploading the consciousness of dying people, we are told what the Heechee look like, and some other bits and pieces. It didn't give me as much of a feeling of a single book-length story with a central destination. Perhaps, it was a fix-up. Perhaps, it's just the ending left me feeling "yes, and then..." Perhaps, it's because the subplot about the Heechee's activities and the subplot about Broadhead's life and changes don't tie together into a single resolution.
We do get to meet Broadhead's girlfriend who was lost in a black hole. But although she does things, there doesn't seem to be an essential reason why she had to be there. Maybe Pohl just tried to piece together old loose ends more than building a full new story - then ended up giving us new loose ends.
The format is different in that here and there a sidebar is interjected with comments from Broadhead's AI (with the persona of Einstein). It's a different twist on narrative and point of view. Different reader may respond to it differently.
I'm inclined to think the book will appeal more to those who want to read another sequel after reading the previous books in the series, than people interested in the book just on its own merits. Certainly, at goodreads.com the average rating for this book is less than either of the two books that preceded it in the series.