Josette,
That is a great question. Sue's book is wonderful and one of (if not THE) best books written so far on the subject of hats, and is a very valuable reference. I wish there was 1 book that covers it all, but there just is not. You need a whole bookshelf and then some.
Dating hats can a real challenge, in my opinion much more so than other garments. There are many reasons for this that i wont go into here. As someone who has been collecting and researching hats for well over 30 years, I still get challenged all the time! The best advice I can give you is to read everything written about hats that you can find...there are a handful of books out there that are good. Some books are a complete waste of your money such as "High Fashion Hats" and "One Thousand Hats" those are both completely useless. There is a lot if misinformation out there, especially on the web. And if you can find vintage and antique hat magazines, fashion magazines, or fashion catalogs with hat pages those can be a great help.
1 thing for sure.....Do not date your hats by comparing them to most of the hats selling on Ebay, Ruby Lane, Etsy and similar websites, etc. 75% or more of the hats on those sites are dated inaccurately. I have counted and done the numbers over the past few years so this is not an exaggeration. And again, the seller may be an ethical, experienced, and knowledgeable vintage clothing or antique dealer, but hats are hard to date unless you really do your homework, and even then....well..there is that challenge again!
Style alone will not date the hat in some cases...you need to know the subtle things about the hat, it's construction, materials, method of workmanship, head size, label and designer. Just to name a few things, there are lots more to look for. You also need a solid knowledge of basic fashion history as well, as hats were often made to go with the new styles of clothing for each year of each decade. And know the designer's or milliner's history as well.
And you just CANT learn it all from a book, no matter how much you read...you have to handle the hats in person to really discern the subtle differences.
It is something that only comes from MANY years of experience, and that will take you time. And you still won't know it all. A day does not go by without my learning something new about hats, and I have studied my whole life.
Best of luck..it is a fun thing to research!
Barbara