There are 64 Horus Heresy books, give or take, depending whether you count the novellas or the graphic novel or the Primarch spin-offs. The point is, there are too many for any normal person to commit to reading. Fortunately, you don't have to. That number includes a lot of extremely skippable short story anthologies, and multiple novels so tangential you won't miss anything if you don't read them.
If it's such a daunting thing you need a guide to get through it, why bother at all? Because at its best the Horus Heresy is an achievement in shared-world history-building unlike anything else, and a gripping saga on a scale you couldn't get if multiple authors weren't working together to build a dementedly labyrinthine backstory for toy soldier games.
