I can help, I've been up to date with almost all Horus Heresy publications. For Garro the order is:
The Flight of Eisenstein (first appearance)
Garro: Oath of Moment
Garro: Sword of Truth
Garro: Legion of One
Burder of Duty / Grey Angel*
Garro: Shield of Lies
*This one has no clear chronological order, but I put here because it makes most sense.
There's also Luna Mendex and Vengeful Spirit which feature characters tied to Garro, which can be chronological after Legion of One.
Then there's Wolf Hunt, which can be chronological placed after The Outcast Dead. (In terms to Garro, I'd say after Legion of One)
Then there's The Sigillite, which is also connected to Garro much like Wolf Hunt. (Also after Legion of One)
Hope this helps.
EDIT:
Good news BrianDavion, you can read the scripts in
The Scripts Volume One and
The Scripts Volume Two. While not completely novels, they should read as a written play.