What I did was took a small standalone addon (meaning not a part of a collection) that did one or two things I liked, for instance YouWereKilledBy (just an example but tthe addon exists), it's outdated yes, but I looked through the code to learn the structure and elements then started researching the newer stuff on wowwiki while looking at the old code, I first got the old outdated mod to the point that it worked on the current build and then used what I had learned from looking at the old code, learning the new code and then developed my own addon.
I don't see the problem with learning from an existing addons code, as long as your not duplicating the code as your own, sometimes just one line of "duplicate" code could be your downfall, highly unlikely but worst case scenerio. You can check out one of mine, they are both simple, the main addon is both lua and xml, the plug-in tho is pure lua, see what they do, where and how . . . then yes check wowwiki and your book for newer, better, less spaghetti code, learn as you develop something small, then take on your own project with what you have learned. I love it and just keep getting deeper and deeper into the code and learning all about it.
I too knew a good bit of HTML and other web languages you should be able to pick it up fairly quickly and easily, especially if your like me and self taught on the web design
anyway good luck and hope to see "author" by your name soon