Here are a couple of resources, right here on tFS.
I suggest that you review this thread, you might find some worthwhile recommendations ...
Books About Fashion, Designers - Recommendations ?
And read all of the old threads in D&C ... especailly the History of Style subforum, if you are interested in vintage collections.
I also suggest that you study historical fashion, because modern collections often are inspired by what has been seen before. Theres' really nothing completely new.
But wish I could offer something more definitive than that but it's just too vast without knowing where your interests lie and the purpose behind your interst.
I just study what interests me ... which is often vintage/ historical in nature. I love the mod era designers who changed fashion so much during the early 60's moving away form the tight waisted, mid calf length skirt into mini skirts, pop art prints, straight little shifts with boots, all very futuristic looking: Mary Quant, Rudi Grenriche (I think I spelled that one completely wrong!), Emilio Pucci, etc. Paul Poiret interests me because he got women out of corsets and into softer, more flowing clothing ... Coco Chanel also changed how women dressed ... softer, slimmer, comfortable ... and that classic little jacket.