well.......there are quite a few books really worth getting if you're a beginner cornet player (the folowing books vary in level of difficulty....the first 1 is for people still learning the fingerings.....as i dont know your exact level of playing, i shall list them all):
Team Brass: Introduces notes and simple tunes.
Winners Galore: A book full of simple to medium level brass music. (Available with accompnimant)
Easy Winners: A bit more advanced than the above book.
Also: How Brass Players Do It......that is full of technical exercies and warm ups.
Good Luck With The Playing!!!!