The answer is "stacks." That is the object of the phrase, and since a person is searching for something ON the shelf, they aren't searching for the shelf itself.
Edmund Kells constructed his own x-ray machine.
Broken Hearts is about a murderer who sends threatening valentines to girls on Valentine's Day.
<em>Summarising is an important skill in academic writing. It enables you to extract the most important points from a text and rewrite them in your own words, in a shortened form. ... Being able to write a good summary also demonstrates that you have fully understood the text you are reading.</em>