Over 100 Pocket novels of TOS have been published and many of these picked up on dangling plot threads, including the Reeves-Stevens' own "Memory Prime" (sequel to the destruction of Memory Alpha in "The Lights of Zetar") and "Federation" (sequel to "Metamorphosis"). Shatner's ten hardcovers, co-written with the R-Ses, have sold infinitely more copies than their regular ST novels. That's not to say that the R-Ses shouldn't write more novels, but the Shatner novels are bought by a huge proportion of ST readers compared to other TOS.
When I started reading Shatner's books, I largely enjoyed them. I thought that ''Ashes of Eden'' was really pretty good. ''The Return'' was entertaining, and ''Avenger'' was ok. It's gone downhill from there. I read Jeff Ayers' review of this book. My jaw dropped. That is as much of the story as I will ever read.