Absolutely essential.
You have a couple choices...
The Annotated Erdnase, which features footnotes and additional text featuring more streamlined/effective applications/handlings of the sleights fro mloads of different card guys.
Vernons Revelations, which is Vernon's ultimate treatise on the expert, featuring his preferred handlings, variations, and applications for the sleights in Erdnase, with loads of photographs.
The original text, in all it's glory. If you're going to go for the basic manuscript, you should get the one with the Martin Gardner introduction, as it's a very nice read and a very apt intro.
Also, the leather bound 'pocket Bible' version from the conjuring arts researcg centre is VERY sexy!
CL