Dale Dubin's book is very good, it is like a workbook, or self-tutorial, and has good pictures and is easy to read.
Marriott's books on the other hand are very intense, but would be a good second reference, after you better understand the elctrical pathways and why waves are going in the directions they do under normal conditions and abnormal.
Conover (Mary I think) also has a very good nursing 12 Lead EKG book.
I know those aren't tutorial's on the web, but Dubin's book is like a tutorial, I learned that way first. And it has stuck with me.