The ever-dependable MozDev has a really clear introduction to multiple columns, “Using CSS multi-column layouts,” at https://developer.mozilla.org/en-US/docs/CSS/Using_CSS_multi-column_layouts/.
The Flexbox syntax has changed so often that almost every current online resource is out-of-date! That said, I recommend Stephen Hay’s article “Learn You a Flexbox for Great Good!” at http://www.the-haystack.com/2012/01/04/learn-you-a-flexbox/, even though it refers to an outdated syntax, as Stephen’s knowledge of CSS layouts is second to none.
The best explanation of the Grid Layout module, at least in regard to the IE10 implementation, is contained in the “Internet Explorer 10 Guide for Developers”: http://msdn.microsoft.com/en-us/library/ie/hh673533%28v=vs.85%29.aspx/.
Read Mark Boulton’s open letter on the subject of grid terminology at http://www.markboulton.co.uk/journal/comments/open-letter-to-w3c-css-working-group-re-css-grids.