There's a thread discussing the differences between Reliquary and the Ref Rules over on odd74 here
Basically, Reliquary will include some minor changes to the "rules", mostly in terms of exact wording, which will also be reflected in an eventual update to the Ref Rules themselves. So my plan is to keep the two in synch.
The main fix to the Ref Rules V3 will be to the wording of the Parrying rule (which a few people have found confusing), but there are a few other spelling/grammar spot fixes too. Holy water will be changed from 5lb to 2lb weight. Werewolves and Giantcrabs get minor tweaks to their combat performance (making them nearer the original).
(Also--since the community is divided on which approach is better--I am considering
an adjustment to the attack matrix which would have it include both coarse-grained and fine-grained advancement schemes).
Those are all change to both the Ref Rules AND to Reliquary (which I want to keep in synch). However, Reliquary will also include a bunch of additional material that will never appear in the Reference Rules including examples, illustrations, and a proper index.
The Reliquary (Delta) file is the most recent release, but it is actually a little bit out of date now. I have a Gamma version which includes everything "up to the minute" but I'm not planning to release it until the whole thing is finalised and at the printer. Your best option, IMHO, is to go with the latest version of the Ref Rules, and to keep on top any any updates.
FYI--there will very shortly be a print on demand version of the DD Ref Rules available thru Lulu, which will include all of the above mentioned updates.
I've already had a "proof" copy (sans the updates) delivered to me.
It combines all three volumes in a single digest-sized booklet, and will include all of the above Ref Rules fixes.
This will shortly be available in all the Lulu binding options, including their "case bound" option. You should be aware that Lulu's case bound option is not a proper section sewn hard back binding; it is simply a perfect bound booklet (with the same glued spine as their other perfect bound books) with a tougher cover. It still might be nice thou.
Hope this helps