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I've found there are two pretty separate types of books. Those that focus on history and design and those that focus on stitches. Most books have at least a bit of both, but I believe the best in each category focus on one or the other.
A few stand out. I'd most recommend Crewel Embroidery in England by Joan Edwards and The Art of Crewel Embroidery by Mildred Davis as the place to spend your money if you're buying to read up on traditional crewel embroidery.
I love stitch books, full of diagrams and potential (I always intend to go through each new book and try each new stitch--somehow it never happens)--some newer and some older are on my list. I like them all.I've found if I don't grasp a stitch from the instructions in one book, another book is likely to present it in a way that will click.
While I was at it I found a duplicate--so it's time for a giveaway!
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Usual giveaway rules apply. Please leave a comment on this post and I'll use the random number generator to pick one on March 7. Not a long time, but I want to get it into the mail before vacation in March. Make sure your comment provides some way for me to get in touch with you. I'll mail anywhere. It'll probably go book rate so patience is advised. Good luck.