Warhammer 40,000, also known as Warhammer 40K, is a vast and expansive universe with numerous books, novels, and stories. The optimal reading order for Warhammer 40K books can vary depending on your interests and what aspects of the lore you want to explore. However, here's a suggested reading order to help you get started:

"Horus Rising" by Dan Abnett: This is the first book in the Horus Heresy series, which serves as a prequel to the events of Warhammer 40,000 and explores the origins of the Imperium and the tragic fall of Horus.

"False Gods" by Graham McNeill: The second book in the Horus Heresy series, it continues the story of Horus' corruption and betrayal.

"Galaxy in Flames" by Ben Counter: The third book in the Horus Heresy series, it depicts the devastating events of the Dropsite Massacre and the escalating war.

"The First Heretic" by Aaron Dembski-Bowden: This book delves into the origins of the Word Bearers Legion and their betrayal of the Imperium.

"Eisenhorn" by Dan Abnett: This is the first book in the Eisenhorn trilogy, which follows the story of an Imperial Inquisitor and his investigations into heresy and corruption within the Imperium.

"Gaunt's Ghosts: First and Only" by Dan Abnett: The first book in the Gaunt's Ghosts series, it focuses on a regiment of Imperial Guard soldiers known as the Tanith First and Only and their campaigns in the Sabbat Worlds.

"Ciaphas Cain: For the Emperor" by Sandy Mitchell: This book introduces the character of Commissar Ciaphas Cain and his humorous adventures in the midst of war.

"The Night Lords Trilogy" by Aaron Dembski-Bowden: This trilogy provides a deep dive into the Night Lords Chaos Space Marine Legion and their dark exploits.

"The Ultramarines Omnibus" by Graham McNeill: This omnibus contains three novels that follow the Ultramarines Chapter and their battles against various enemies of the Imperium.

"Ravenor" by Dan Abnett: This is the first book in the Ravenor trilogy, which is a spin-off from the Eisenhorn series, focusing on another Inquisitor and his team.

These are just a few recommendations to get you started, and there are many other great books and series within the Warhammer 40K universe. Feel free to explore different factions, characters, and time periods based on your interests.