Looking for Helminthology textbooks

[Francois Dreyer]

Does anybody know of any good Helminthology textbooks I can get for the lab. 
Most of the ones I've got at the moment dates from the  30's and 60's.

I am especially intereseted in books that can assist me in identifications. 
I mostly deal with sheep, cattle and horses but sometimes get strange worms 
from seals, primates and different kinds of avian species.

Thanks.

Francois Dreyer
Parasitology Departement.