My personal favorite is the basic gold with the Shadow Keepers and Solar Watch being close behind. I'm certain you'll have no trouble finding all the lore you need, but if you ever have a question I'd be happy to help.
Aside from the usual wikis, lexicanum, and codex. The best start would be the "Watchers of Throne" series by Chris Wright.
Unfortunately Custodes lore from the older books can be quite contradictory as much of it was written before their revamp a few years ago.
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Unfortunately Custodes lore from the older books can be quite contradictory as much of it was written before their revamp a few years ago.