SoB book recommendations

My friend has recently been getting into the lore of 40k and has become interested in the Sisters of battle. Being as unpopular as they are it's hard to find books to recommend about or including them. So I thought I'd ask my fellow Archons if you had any recommendations.

Sadly there isn't all that much to read about the Sisters outside their codices.

There are two novels: Faith and Fire, the sequel Hammer and Anvil, also its audio prequel Red and Black.
And then there is the Daemonifuge comic series, but it deals more with the Inquisition and Chaos than with the Sisters themselves.

They appear quite a bit in Duty Calls, by Sandy Mitchell. It's one of the Ciaphas Cain books and is pretty good

Hmmm thanks for the suggestions guys. I'll take a look at those books.

There also used to be an old comic called Warhammer Monthly who printed a stoey called Daemonifuge. It got I think to the end of volume three or four. Unfortunately the overall story arch never finished completely because the comic shut down (with about a single issue's notice) but the trade paperback graphic novels of the first few volumes may still be findable out there. Somewhere.

The novel 'Grey Knights' also features sisters fairly heavily.

Also, there is a solid Sister story in the 'Let The Galaxy Burn' anthology.