I've heard some people call Horus Heresy warhammer 30k since it really happense before 40k. I was wondering if there are any books that talk about the story even before Horus Heresy. Basically, is Horus Heresy the oldest series in chronological terms or are there series before that?
The "Horus Heresy: Primarchs" series covers the origins of several Space Marine chapter primarchs (e.g. Roboute Guilliman: Lord of Ultramar and these cover events prior to the Horus Heresy, such as the Great Crusade, and are therefore set chronologically earlier than the Horus Heresy series itself. However I can't confirm if there are any novels set even earlier than this period.
The Horus Heresey series is the earliest 'dedicated' series of novels - however snippets relating to the age of strife, unifications wars, birth of the emperor etc can all be found in various Heresy era novels - I'd recommend the Lexicanum (Pre-Imperial Times) for better information and a list of cited sources.
Most articles you'll find online are based on inference or speculation with no real hard canonical line from games workshop yet.
The Horus Heresy is basically a massive prequel to the entirey of of the lore which takes place in 40k. There is no substantial series of novels which covers anything that takes place prior to the Heresy.
There are some really interesting articles and wiki entires on the web which explain some events that take place before the Heresy:
But not much else.
Graham McNeil's short story "The last church" takes place towards the end of the Unification War, so it would be even before the Great Crusade. It is not a typical Wh40k story, because it depicts the conversation of the last priest on Earth with a mysterious character called "Revelation".