Grid systems are such an important part of design, but often one that in the busyness of multiple deadlines, get overlooked or not done at all. I am of the opinion that they are not necessary in every case when the designer is using a more organic approach or is very comfortable with the layout. I think they are viewed sometimes as being too restricting. But, in a lot of cases, a strong underlying grid can expand the design possibilities and add structure and organization. I think this is especially important when text is being used so that the legibility is fully considered.
I was recommended the book Grid Systems by Josef Muller-Brockmann.
Aside from being written in both German and English, the book is very clear and thorough. It takes a fairly traditional and structured approach to the subject matter. But, it breaks down all the information and gives may examples of how they can be used.
You can tell instantly by the imagery and typography that the ideas in the book are taken from the 1960s, but the content really has never gone out of style.
It is a great reference if you are ever stuck with a layout or need ideas on how to organize complex or a multitude of information. I am a solid believer in classic information and approaches to modern design being respected and understood. Once you have these good foundations on basic elements and structure, breaking the rules is easier and more effective.
I would recommend this book on the shelf of any designer. If you know where to look for the information, it can help make sense out of some of your text and visual elements.