I second Darryl S comments on Paul Matthews books - they are essentials -

Loading the BP Rifle Cartridge - ISBN: 1-879356-20-1
Shooting the BP Cartridge Rifle - ISBN: 1-879356-38-4
How-To's for the BP Cartridge Rifle Shooter - ISBN: 1-879356-41-4

Another great book is SPG Lubricants BP Cartridge Reloading Primer by Mike Venturino & Steve Garbe, available from SPG, a site loaded with good info on Black Powder cartridges and the rifles that shoot them.

SPG Black Powder