"Star Trek: The Next Generation Companion" was something of a disappointment to me.
It's not that I didn't enjoy reading how certain special effects were done on a particular episode, or how casting for a particular part was done. This companion book contains many interesting behind-the-scenes facts and was, overall, very enjoyable to read.
The disappointment I have with the book comes from the many errors I saw in the episode summaries. You would think that these summaries would be the easiest part of the book to write. Just watch the episode and then write the summary. It appears that whoever wrote these summaries did so from memory. Unfortunatley, that memory seems to be slipping.
One example is the summary for the "Star Trek Nemesis" movie. The summary makes it seem like Captain Picard does not realize that it is Data who has freed him from the lab on Shinzon's ship until after they leave the lab (supposedly he thinks he is being taken by B-4 to Shizon). However, Picard realizes that it is Data while still in the lab when Data renders the guard in the lab unconscious.
In addition to inaccurate summaries, there were also summaries that, while technically accurate, failed to completely describe the episode. In other words, they were too short. "Deja Q" is an example of this. There was quite a bit that happened in this episode, but the summary makes it appear that this was nothing more than a simple, basic show.
Although there was a lot more that could have been done to make this companion book better, it is still one of the best one out there for the TNG series.