EDI is an old enough concept that you should be able to get plenty of the basics from a used book store and save $$$. (Go for a new book only if you need info on XML or e-commerce concepts.) If there is an "EDI for Dummies" book, I think that would be a good starter.
After you browse whatever book you buy, find a live person to explain things: peer, mentor, computer group, or trading partner (with whom you will be exchanging EDI documents). Sometimes a college or local school district will have a Continuing Education course on the subject and may or may not fit within your budget and time constraints. The key here is experience: someone who has done it before.
EDI is standard only in the most general sense. Because EDI is so broad a standard: 1) only a fraction of the available data fields for any particular document (e.g., 820 or standard version, e.g., 3020) are used and 2) one trading partner may differ from another as to how each implements the standard. Big trading partners often create "implementation guides" that give specific information on how they will send or how they expect to receive EDI data. Check with your trading partner.
Communication/transmission/exchange of the documents won't be in any standards book, but is a critical element of doing EDI business. If you are a novice in this area, again, try to find a live person to help you with it.
Hope this helps.