Sonnets

I'm interested in trying to write a crown of sonnets. As I like to write in free verse I wonder if I'll have to call it a crown of 14 liners instead. I'm going to research whether a sonnet really, absolutely, has to rhyme. I've just ordered Tony Harrison's Selected Poems to see if his School of Eloquence sequence of sonnets can give me some answers. That Amazon one click is lethal!