Originally Posted by HanyaanFaery
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.
Seconded on the Vogue Sewing
book. I have my mom's from the 60s, and while it's missing the sections on sewing newer fabrics such as lycra it's still incredibly valuable; whenever I'm stumped on something I just go there.