Here are my favorites from this year, all worth having on your bookshelf:
If you're getting a gardener/designer just one book, get them this one: Fine Foliage: Elegant Plant Combinations for Garden and Container (St. Lynn's Press, 2013) by Karen Chapman and Christina Salwitz, Seattle-based designers. It's the best I've encountered on plant combos based on texture and color. It belongs in everyone's library. My full review is here.
For a historic view of British garden & design style, you should definitely order Royal Horticultural Society Chelsea Flower Show (Frances Lincoln Ltd, 2013) by Brent Elliott, the RHS's archivist & historian. You'll learn something new every time you pick it up. Full review here.
Something a little different, but it's a reference you shouldn't be without: The Professional Designer's Guide to Garden Furnishings (Timber Press, 2013) by Vanessa Gardner Nagel, APLD. It's all here: fences, containers, fabrics, stone, synthetic materials, and much, much more. Full review is here.
And for a close look at some classic American garden style, think about Private Gardens of the Hudson Valley (Monacelli Press, 2013) by Jane Garmey. Twenty-six gardens are profiled in this book, from classic English-style gardens to the very contemporary. Beautiful photos and a lovely survey of current styles in the Northeast. Review is here.