If you are looking for current gameplay rules rather than vintage lore, Games Workshop has transitioned to a "Digital First" approach for many resources. Warhammer 40K Codex Imperialis
Ethically and practically, the answer is nuanced.
: If you are only after the specific lore or "fluff" rather than the full book layout, the Lexicanum offers exhaustive summaries of the information contained within these classic volumes. A Note on Compatibility
The book opens with a stirring description of the Master of Mankind. Unlike modern lore, which often delves into the Emperor’s origins or the Horus Heresy in detail, the Codex Imperialis treats the Emperor as a distant, mythical god-figure. It emphasizes the religious fervor of the Imperium. The description of the Golden Throne and the sacrifice of psykers creates a palpable sense of dread.
Unlike the Codex: Space Marines or Codex: Chaos , which focus on a single playable faction, the Codex Imperialis was a hybrid product. It was originally released in 1993 as part of (the box featuring a Blood Angel versus a Genestealer). Later, in 1995, it was re-released as a standalone softcover book.