Professor Li is the authority on the 24 step form and I recommend his book "Tàijíquán" which gives comprehensive instructions with pictures for the 24 step form as well as the 81 and 42 step forms, and the 42 and 32 step sword forms. It comes with a dvd showing demonstrations of these forms.
He has also produced teaching videos on the 24 step and 88 step forms amongst others. You can buy both book and dvds from the Tai Chi Link website. They are also sometimes available on Amazon.