First things first- the previous reviewer's claims about the Victorian photo plates are absolutely, 100% spot-on. The plates are hilariously inaccurate, and make for weird distractions in an otherwise valuable costuming resource.
Besides that admittedly embarrassing oversight, I have no idea why this book would warrant one star. If you can look past the photos (please, god, look past them), there's a wealth of dense information in this book pointing to primary sources and often so granular as to name specific years and dates. Medieval fashion can be a tricky thing to research- there's so little documentation compared to later periods- so I absolutely appreciated this book as a look into the specific evolution of fashion during the so-called "earliest times." This should by no means be your sole resource for British costume, but I'd still argue that it would be an invaluable addition to your library.
Do you like hennins? BY GOD there are so many diagrams of hennins. You will drown in them. It's pretty great.