Board Thread:Skyrim/@comment-108.34.49.17-20130122190224/@comment-5917342-20130122200753

Yep the book is actually in Kagrenzel. The path you couldn't reach is the exit of Kagrenzel. Now you have two choices:

1.Get in Kagrenzel, touch the light ball and fall. You'd better save the game before touching the ball in case you touch anything and game over during your free falling. Make sure you fall in the water. Then find a path uphill. Follow the quest marker and you'll find the book in a falmer chest  on your way to Stony Creek Cave.

2.There's a high ground oppsite the waterfall. I managed to get into the waterfall from there by using Wu Nah Kest(Whirlwind Sprint). Follow the path to Kagarenzel and you'll find the book.