The html books are trying to teach html, so they are using upper case to bring the tags up front (while masking the rest). I started out doing html in all upper case, but it wears on your eyes after a while. I find upper case words much harder to read as a block - it's like shouting. Lower case is much easier to follow over the long haul.

One exception to the rule is the DOCTYPE tag - some browsers insist that it be upper case.