ferrets require a very high quality diet, which can get expensive. they need a fairly large cage, which can be expensive. if kept singly, they need lots and lots and lots and lots of human interaction. even if kept in pairs/groups, they need at least four hours play time DAILY. on top of routine shots and vet visits, there are common health issues that can be extremely expensive to fix. ferrets are predators. depending on the animals involved, they can be great friends with dogs or cats (NEVER UNSUPERVISED), but they will kill small animals such as chinchillas, gerbils, rats, rabbits, ect. ferrets have a musky smell to them (i rather like it) no matter how well they are cared for. feeding a high quality diet, cleaning the litter box AT LEAST once a day (i do mine three times, but i have five ferrets), and cleaning their bedding/cage regularly can help keep the smell down.
i second ferrets for dummies, it's a good book. also,
www.ferretvillage.org has been rather slow lately, but there's still lost of good info on health, diet, care, behavior, ect there.
good luck!