Seth Godin said to give each page on your website ONE JOB. Only ONE THING you want the surfer to do.
I have just spent about 3 minutes on the URL you link to there, and I cannot see your lecture notes ebook.
Might I politely suggest you do a landing page for the notes and link to that? Or at least make it obvious? Or maybe provide a direct link for those that wish to purchase the notes?