Don' t really know of any good books on the subject, but I' m sure there are some out there. I learned muzzleloading from my Dad, and from good ol' trial and error. I would say your best bet would be to seek out one-on-one assistance at a local hunting club, or even try asking at a local gun shop.
If the gun looks to be in good shape then it probably is. Inspect it thouroughly before you shoot it. Pay attention to the bore. Rust or pitting is a sure sign of neglect, as muzzleloaders need prompt and thourough cleaning because black powder and subs are very corrosive. If you' re not sure, take it to a competant gunsmith for a checkup before you risk yourself.
The next thing I would do is determine the pitch of the rifling (twist rate). The twist rate will determine the types of projectiles the gun will fire the best. To figure out the twist rate you can use a cleaning jag on the ramrod with a tight-fitting patch. Start the jag/patch and mark the ramrod toward the end inline with the front sight. Then slowly push the ramrod down the bore allowing it to follow the rifling. When it bottoms out determine how much the mark rotated as precisely as possible. Also measure how far the rod went into the bore. Divide the length by the amount the ramrod turned to get the twist rate (ex. if the rod rod goes 33" into the bore and turns 1/2 a turn then the twist rate is 1-in-66" ) The actual number need not be super precise, but if you are careful it will be close enough to determine the preferred projectile for the gun.
A general guide is that a rifle with a twist rate of 1-in-48" or greater will shoot patched roundballs most accurately. Between 1-in-32" and 1-in-48" will likely best shoot lighter (shorter) conicals, certain lighter sabots, and should shoot roundballs OK as well if closer to the slow end. Anything faster than 1-in-32" is designed for long heavy conicals or medium to heavy weight sabot (for the caliber) bullets. Most " modern" muzzleloaders are rifled in either 1-in-28" or 1-in-48" , with 1-in-48 being more common in the " traditional" looking rifles like I suspect yours is. I' m guessing its a replica of the " Kentucky flintlock rifle" which were originally made between about 1750 to about 1850. The Kentucky rifle is, to my knowledge, the first truly American rifle design and was a major contributor to early American history.
Anyway, congrats on your new rifle and welcome to the addiction.
Mike