Free, downloadable titlesThere are a number of millinery books that have been digitized and are readable online or downloaded in a variety of formats. The Internet Archive has several -- keep in mind that most of those books are quite old, dating from the 1890s to the 1920s. Also, a search of the keyword "millinery" will yield results that include socioeconomic studies of women in the trade of millinery, which you may or may not find interesting.
Millinery by Charlotte Rankin Aiken, 1922, 188 pages
Concerning Millinery, by Kate J. Giblin, 1902, 73 pages
A Complete Course in Millinery, by Julia Bottomley (ed.), revised ed. 1919, 167 pages
Complete Guide to Millinery, by Margaret Kitnzel & Mary M. Lunt, 1915, 54 pages
Home Millinery, Madame Margariete's Manual, by Margariete Creamer, 1920(?), 15 pages
Make Your Own Hats, by Mrs. Gene Allen Martin, 1921, 111 pages
Le Petit Maître, by Marie Melcher, 1895, 68 pages (despite the title, the book is in English)
Modern reprints of vintage booksBramcost Publications has several titles -- 3 dozen at the time of this writing. I have their republication of the 1944 book, How to Make and Trim Your Own Hats by Vee Powell. They are pricey; a 23 page 8.5x11" book of crochet patterns is $14. On the other hand, finding an original vintage copy on ebay would probably run more than that. They also sell a variety of other vintage reprints, including sewing, beauty and entertaining.
And speaking of ebay... Dakota Prairie Treasures sells the same sort of titles that Bramcost does, spiral bound (so they'll lay flat when you're working), and in some cases, the same titles, like How to Make and Trim Your Own Hats. The prices are similar, with DPT being a bit less expensive. DPT also has some more recent titles from the '50s and '60s. They have three pages of items listed under Millinery Instruction in the shop right now. I have not yet ordered from them, so I can't comment on the quality of the reprinting, or the sturdiness of the spiral binding.
Modern millinery titlesA quick search on Amazon yields a number of modern titles. If you find a title that sounds good, but you're not certain you want to commit to the cost, check your library (or inter-library loan). Or there's always Half Price Books, Half.com (an ebay company), and ABEBooks.com -- all excellent sources of used books. You'll find some overlap between Amazon's used booksellers, Half, and ABE.
I don't currently have any of the modern books, but I did pick up a copy of Felt Jewelry by by Teresa Searle because several of the projects looked like they'd be good for millinery trimmings. Plus, I enjoy felting.
As I find titles, I'll try to review them and share my thoughts here.