Quickly finished "Spear of the Emperor" by Aaron Dembski-Bowden, and first published with a special edition that included decals for the Emperor's Spears space marine chapter. I bought the 2020 trade paperback edition, and enjoyed it.
This story was written before 9th edition came out, having the chapter included on page 81, although I couldn't find the system it take place in, Elara's Veil, on any of the maps. The story is a telling from one of Amadeus Kias Incarius' helot's viewpoint. He is a Mentor Legion space marine who has been sent to discover what is happening with the Emperor's Spears and the Celestial Lions, after the failing of the gene seed of the Star Scorpions and their disappearance into the Void. The ship manages to make it across the Great Rift, and besides the loyal chapters they interact with, there are also The Exilarchy and The Pure encountered, and not only do battles ensue, but also a capturing, escape, a reincarnation of Amadeus, and an ending that brings into play perhaps questionable motives of the Mentor Legion, and maybe even in league with the Inquisition. The stage is set for at least a further novel, if not more. I am looking forward to the next book when it is released, and apparently it is in process.
From a writing perspective, there were three items I was less impressed with ... a scene in a tomb which seemed kind of blah, a star cruiser crashing into an enemy's ship kind of out of the blue, and a perhaps less than stellar move through Amadeus' reincarnation and his helot's healing. However, this book contributes an interesting perspective in traveling into the nasty side of the galaxy, an increase in Mentor Legion information, a good introduction for the Emperor's Spears, and some writing of a scene where the Mentor Legion's ship is boarded that made me wonder how the author had experienced the scene described. As it turns out, the author had interviewed firemen to be able to write the picture of what happens.
So in general, I recommend picking this story up, it was a solid page turner. It has even got me wanting to paint some of my Mentor Legionnaires, and a desire has returned to creating my own Space Marines chapter that my Mentor Legion figures support.