The earliest book in the timeline would probably be The First Heretic, as it covers the events that turned the Word Bearers to Chaos, which then lead to the events of the the Heresy, including Horus Rising and the subsequent False Gods, Galaxy in Flames, Flight of the Eisenstein and Fulgrim novels. I think the rest of the HH series, apart from Book 14 The First Heretic, is set some point after the Dropsite Massecre.
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