I don't have any idea what capacity you worked Endo prior to becoming an RN? So I will just assume you really know nothing about Endo nursing.
I have only floated to Endo, (but many times) over the past 13 years. I am ashamed to say I have never looked for sites or resources. The regular endo nurses have been there a long time and my gut feeling is they have learned a lot through on the job training. Which means as a new RN you have to ask a lot of questions. Tell the doctors you are new to endo nursing and ask, ask, ask. The doctors, no one you work with, will expect you to know everything and no one should make you feel dumb if you say I don't know what that is, please tell me about it.
Ask the tech, the sales reps, anyone who has any dealings with endo nursing, questions.
Just remember when you are in the room doing the procedure the patient may, can, hear you talking. Be a little discrete. Some questions should be asked after the patient has left.
Ask again here if you have more specific questions. And of course the Internet is a great resource for information about GI diseases.