Most Helpful Girl
One thing I always found helpful when getting serious about any subject is to look up university reading lists. They don't always have the best books or w/e but it usually serves as a good jumping off point.
A quick google search for 'university reading list fashion' turned up:
Obviously this isn't catering specifically to what you're looking for (there are some books about sewing techniques, for example, I'm sure you don't care about) but it could provide some good leads :)
also StyleCaster, an online magazine I have never heard of before, has an article titled "the Best Books to Read If You Want To Work In Fashion" - sounds good I guess?