Other than some of the ones mentioned in this thread, I like Vogue Sewing. A lot. It has instructions with diagrams on how to do pretty much anything you'll ever encounter with a sewing machine, an extensive alteration/correcting pattern fit section, and a huge glossary of every sewing term pretty much ever. You can get any edition and it'll still be useful. Mine is from the 80s, and it's still entirely accurate with the exception of what "modern" fashion is. (In which case it's good for a laugh.)
I also think I'd be really sad without my various costume history books. It's really useful for series with vaguely historical-inspired outfits to see what the actual clothing they're based on looks like. It's amazing how often I end up flipping through my Kyoto Fashion Institute History of Fashion two-volume set, and I have this old History of Costume Dover book that wins. It really is amazingly helpful at times.