I guess I don't really qualify as a new banknote collector anymore, yet it is important to make sure that if you want to take this hobby seriously and collect lots of banknotes, acquired knowledge over time will only come from some serious referencing.
I have found during the last couple of months that the best way to gain knowledge about banknotes, their value and different issues is to own a Standard Catalouge of World Paper Money. The Vol 3 would be your best first buy as it deals with 20th century notes to the present day, depending on which edition you can lay your hands on.
These are really like the paper money collectors bible, you should not leave home without it.
Checkout these books for banknote collectors on offers from Amazon: