In my opinion, you’re having problems because your title is far too broad. Perhaps focus on a particular period in time, country, designer or studio.

I really think you should try to narrow your focus a bit. It’s kind of like trying to write about the use, effect, and value of acting in movies. Maybe think of a particular era of graphic design that interests you, or a particular type of design (books, posters, something).

The trio of Wayne Booth, Gregory Colomb, and Joseph Williams have excellent advice for focusing research topics. See their book The Craft of Research, or their edition (7th) of Kate Turabian, A Manual for Writers.

Graphic design without type suggests an approach.
Either by removing type from various media (one artist has done that with street scenes), or “beyond language” infographics (eg airplane emergency instructions).
There are grey areas that may shed some light on the purpose of type in graphic design, on the border line between a text message and an icon—compare No.19 and 20 here:
http://typographica.org/001009.php

Also related, Adobe’s switch from icons to letters for their Creative Suite GUI links, which appears to be bucking the trend.
There’s some interesting theory behind the Apple interface, which I touched on in an article on OS X:
http://www.shinntype.com/Writing/DeeperIn.pdf

yeah your topic is way too broad. think of some use of type that you find fascinating, think about why that is, and delve deeper. the narrower, the better. then you’ll find a context for it (broaden your scope), but you need to come to a narrower and more exciting (to you) topic first.