The definitive work on the Illinois Watch Company is the "American Pocket Watches, Illinois Watch Company Encyclopedia" put together by Bill Meggers and published by Roy Ehrhardt's "Heart of America Press" a few years ago. This 420 plus page spiral-bound volume weighs three pounds and carries an equally heavy sounding $80.00 price tag. Heart of America Press (APWE2) Link to Illinois Encyclopedia Listing
This book is a formidable volume and it takes a great deal of patient study to fully comprehend it. However, if you're a serious Illinois collector it is indispensable to getting the facts and keeping them straight. Illinois is a complicated company to understand, we're still learning about it every day and that's what makes this hobby so fascinating.