I was given In Your Garden by Vita Sackville West years ago before I even had a garden; she taught me so much. The book is a collection of her Observer columns from the 1940's / 50's. I have since got hold of In Your Garden Again which is the follow up. Beautifully written, as one would expect, but also absolutely cast-iron tips and advice.
I also really love Robin Lane Fox's Variations On A Garden for the same reasons. He is also quite entertaining.
The Morville Year by Katherine Swift. Although sometimes a little too mannered / consciously erudite but it might just be that I am envious of her life and garden.
Dan Pearson writes very well on a blog.
Any other suggestions for beautiful garden writing?