not too short
"Section One" won't help much when it shows up in a search results list. Section one of what?.
Better: <title>Section One of the Modern Music Guide</title> or even better <title>Time Frame (the Modern Music Guide, Section 1)</title>
not too long
because only 60 to 80 characters are displayed in many window titles, menus, etc.; you can give a more detailed/pleasant title in the top level heading of your document, e.g. The Modern Music Guide - Section One: When has the Modern Time Commenced, Musicwise?