Gertrude Jekyll was an influential British garden designer, writer, and artist. She created over 400 gardens in the UK, Europe and the USA and contributed over 1,000 articles to Country Life, The Garden and other magazines.
The book is out of copyright and available on archive.org.
It's quite dated, so if you're looking for a guide on planting & growing roses, you'd be much better off looking for such info on the net, or in contemporary print guides. What advice is given in this book, is not very detailed. The only useful bit of advice I found was that if you use a stake then the bottom end of it should be charred before putting it in the ground to prevent rot (or you could just coat it with wood impregnant). Also, the book was written in a style equivalent to that of a contemporary hobbyist blogger - there's quite a lot of the author's personal opinions and preferences.
Also, the book has a lot of photographs, but they are black-and-white and it's difficult to make out anything in garden photos. But it was interesting to see what tea roses looked like, as opposed to hybrid teas we know today: pale-colored, messy, unsymmetrical blooms, combined with worse health - no wonder they were replaced by HTs.
The only value of this book is historical. It's interesting to take a look at what roses were grown more than a century ago (those poor, poor people that didn't get to enjoy Old English/Austin type roses we have today) and the way it was done. Also, it was fun to notice familiar rose names that have survived since then, even some that I've got or plan to get myself:)