Definitely the janet Arnold books
I disagree with a few conclusions of the very early costumes but that is with the advantage of what.. 30 odd years of further research
Much of wich is now very readily available.
I also adore the Fashion book from Kyoto Costume Insitute. mainly for inspiration with Arnold et all as the means to understand the construction.
The "In Detail" series from the V&A are wonderful also for inspiration but also for the details of the fabrics and trimmings etc.
Alcega, the 16thC Spanish book meant to show tailors how to lay their fabric. With Arnold III you can refer back and forth to see how the final pattern pieces look and how you cut them.
Blanche Payen's History of Costume but only the first edition. I am boycotting all reprints that do not include the patterns. They are not of the same garments ast Arnold so they are a further amazing research tool.
Nora Waugh's Cut of Women's Clothing, Cut of men's Clothing and Corsets and Crinolines. She includes contemporary texts that describe how the clothes were viewed but those who wore them and those who wondered what this fashion malarky was all about
But also patterns as well as cutting diagrams of the various times.
Harpers Bazaar 1860-1903 or so. It's a Dover reprint and is comprised of only fashion plates from those dates from said periodical. Great for really getting to grips with the insane changing fashions of the Victorian era.