You can buy "Shadows of Treachery" It has this story plus others. I was in a local game store recently and they had a physical copy of his. Hardcover and the price was $24. Black library and gw are getting to almost revolting levels of greed in regards to their horus heresy series.

Many Horus heresy books have been showing misleading (and I'm being nice) release dates lately, not even showing as already owned if you had purchased the "true"/initial release. This is one of them and this time I fell for it as I didn't remember this tittle. Weird business practice (I'm being nice again...)