It really depends what you plan on doing with the Assembly language. For a good, indepth introduction into the language, I would recommend
Step By Step, The Assembly Language by Jeff Duntemann. He's a very good writer, and the topics in the book are relatively up to date. Another good, out of print book, is called
Assembly Language Programming, cowritten by Peter Norton (founder of Symantic). You actually walk through the production of a full blown disk editor in Assembly. Very cool stuff. If you have no desire to buy or check out a book from the library, there is a free online book called
Art of Assembly Language Programming. It's a very respectable book; just look
here. Online tutorials will likely not be as good as published books simply because it's harder to keep someone's interest in assembly, since you don't have as many bells and whistles, as opposed to a language like Java, C++, C, Pascal, VB, etc.