30k Noob advice

As a 'lapsed' 40k player of many, many years I recently got back into the hobby via the Calth boxed set.

I've settled on Imperial Fists as my Legion, and the army list is slowly coming together, but my question is this ?

I've already got all three 'red' books and will purchase the new version (S) as they become available, but would any of you care to recommend an order to buy the main volumes in ?

Am I better picking and choosing individual volumes or just going chronologically ?

I'll probably get them all as funds permit, but are there any must -haves ?

Thanks in advance

I my opinion, the rate at which these rules keep getting updated/modded, means I can't really recommend any of the black books as must haves.
I personally love them and bought them as they came out so have really enjoyed reading them and having the rules to study.

The one with the campaign system using supply points, I think that's 4 (at work, can't check), is a good one to have for that. Book 3 has the Imperial fist background though.

If you've got the red books already, I say just begin at the beginning, all the books are fantastic; well written, well presented and well made. That said, if you just want Imp Fists, then go for book 3, the first is all traitor, the second traitor with the Istvaan legions, the fourth Knights and Solar Auxilia and the fifth has Smurfs , Bible-bashers and the Warlord Titan. They all have ad-mech.

I would just buy the one tied to your legion for the fluff if that floats yer boat.
They seem to be good aboot wanting to make "one book to rule them all" for the legions.
No matter what way you slice it, yer gonna continue to need to buy the books (well...you dont NEED to depending on your view of the law, not condoning that but with those prices its a very common practice) until they got every single legion done.

Its an expensive game. If you are not prepared to fork over the dough or steal them id wait.

I've bought all the books and quite frankly they are the best hobby purchases I've made in the last 10-15years easy. In my opinion (whatever it's worth) they're right up there with the classics like Slaves to Darkness and Ere we Go. Truly brilliant stuff. It's not just the background which is fantastic the rules are great, the Victory is Vengence campaign is my personal favourite. Although there's much much more I can't recommend them highly enough. I liked them so much I even bought the special edition so I could have the case and I don't normally buy the special editions. Oh and they're LEATHER bound!!

Take your time they are pricy and get them in order. They add so much depth to the Heresy

Thanks for the replies, I've gotten a better idea of how to proceed now, I'll probably start investing as soon as my next payday rolls around.
Extra campaign rules and scenarios sound great, I've a friend doing Alpha Legion, so anything that gives us an excuse to do a min-campaign is very welcome
Thanks again