The only one I own so far is The Ultimate Sashiko Sourcebook, which I bought after writing this post. I think I'll have time to practice some sashiko stitching this weekend, and this book has tons of great designs to try out. I'd also like to try my hand at dyeing fabric to use as a base for some embroideries, so Tie-Dye: Dye It, Wear It, Share It and Shibori for Textile Artists would be good to have. Lastly, Storyland Cross Stitch was featured on quite a few embroidery blogs a while back and has been one of the first books that actually made me want to try out cross stitch.
My only problem is finding enough bookshelf space (don't get me started on the non-craft books on my wish list!).