I've changed a lot about my design approach and process after reading the Manifesto of grid-based layout -- Grid Systems by Josef Muller-Brockmann.
I've expanded this knowledge further with Grid Systems: Principles of Organizing Type by Kimberly Elam and got some interesting inspiration from the practical examples in the book Making and Breaking the Grid: A Graphic Design Layout Workshop, which you can get as a PDF here: http://rapidshare.com/files/322538310/Making_and_Breaking_the_Grid.pdf
The Vignelli Canon is also a must-read in my opinion. It's available for free from the author here http://www.vignelli.com/canon.pdf
I'm also trying to read The Elements of Typographic Style by Robert Bringhurst, but I find it hard to read since I don't have a printed copy of this (just a scanned , hate that).
So, any other reading suggestions from you good people? Any interesting blogs/articles/pdfs on this subjects that you would like to share?