I made a Black Legion specific tactica
thread a while ago. Might give you some ideas, it didn't get very far mind you. Neither of the legion specific threads did.
As for what books you need?
IA:13 from forgeworld is less needed now, but still a good book to have (if you can get it). Otherwise you just need legions and the base dex. Well, maybe traitor's hate if you 're not using
FW.
A
BL Black Crusade detachment (the traitor's hate decurion) and a
BL CAD are pretty similar. The only thing you get, really, is the free boon roll since you hate everybody by default and our Termies and chosen are ObSec in a
CAD too.
If you want ObSec either works. The
CAD is pretty much mandatory for
FW while being more customizable and potentially cheaper, while the Crusade has ObSec bikers and a free boon.
Alternatively you could just take the warband on it's own, since loosing that free boon isn't a big deal. You're really only interested in getting ObSec.
In that case you only need traitor Legions since the Traitor's Hate formations are reprinted in Legions.
In other words you only need legions and the dex, unless you specifically want a renegade knight.
FW has free pdfs for actual
csm knights which aren't bad either. I wouldn't say you need either knight though.
So Black Legion pretty much boils down to whether you want to use the T1 deepstrike, have ObSec or bring
FW.
I like my forgeworld units and the flexibilty of a
CAD so I'd stick to that. The speartip enables some interesting tactics, but you're taking a gamble. Plus I still haven't quite warmed up to the large formations.
So as far as I'm concerned, unless it's for a fun game, I'd stick to a
CAD with a pinch of formations as necessary, like the cabal.
If you plan on bringing a TAF or Raptor talon though I'd really consider the speartip. T1 can be a great help for both by virtue of possibly hitting stuff before they can activate buffs, fly etc and crusader helps the TAF move after firing.
Although it's not bad even just for mitigating reserve rolls, doubly so if you manage to squeeze in a comms relay or so. And both formations are pretty good already.
Oh and for the black mace, a daemon prince makes it AP2, that's why it suddenly becomes awesome. S8 is the delicous frosting on the cake as that elevates it into Instant Death territory for most units, plus the prince can now take on vehicles pretty reliably. Nobody uses the mace on anything but a prince because of the otherwise
AP 4. And if you're fishing for iron arm then you may as well stick to the force maul.
**edit**- actually nvm the S8 part That's whishful thinking... I got carried away by ZebioLizard2 XD. It doesn't give +2 strength, but it's still amazing. Although you can still get S8 by other means as a prince.