Yes, I know it's vital to have a teacher -- I do!!!
My story is that I have been singing for over 10 years without one, and now that I got a teacher, she told me that I sing too far back in my throat, that I resonate to my nose a lot, and just in general that I have a great voice and wrong technique. I have difficulty understanding how to do what she tells me to do because I can't seem to grasp it yet, what the correct way of doing it is. If I could just get it once, I would practice my butt off to do it -- but every week she tells me the same thing and every week I keep doing it wrong. Please check out my video on YouTube -- http://www.youtube.com/watch?v=gfDgh_hcelY and feel free to comment!
If anyone can recommend any books that describe proper singing technique, I would VERY MUCH appreciate it. Amazon is full of them, but I can't just pick, you know? THANK YOU SO MUCH!!!!
The best book I've ever found is Seth Rigg's book "Singing for the Stars". It contains a comprehensive set of vocal exercises which work together to improve the voice. It also includes audio cd's to practice the exercises. It totally transformed my voice! Seth also has new program called Singer's Advantage. Another good program is Brett Manning's Singing Success, which I'm currently using. Brett was a student of Seth's and added new exercises to his technique. Just do the exercises you'll see amazing results.