As Deus Mortis has said. To rephrase it, you need the Legiones Astartes Crusade Army List, no matter what Legion you run. That has the rules for Praetors, Tactical Squads, Contemptors, Support Squads; everything that isn't a unique Legion unit. After that, you'll want one other book for your Legion's rules; Isstvan Campaign Legions for a codification of all the Legion rules so far other than Ultramarines and Word Bearers, which are in Book 5: Tempest (I think Word Bearers did have rules in one of the previous books, primarily for their non-Chaotic units such as the Ashen Circle, but were updated, expanded and codified in Tempest).
I've been building what I can from my kits (still waiting for my upgrade packs, so a bunch of Marines are hanging around without pauldrons, heads or weapons) and the kit is beautiful. Extremely nice plastics.