Buy directly from the publisher’s website. Only O’Reilly doesn’t sell books, they have only an expensive subscription. Manning puts your name and email at the bottom of every page. Apress and Packt for example don’t. There is no DRM. After Thanksgiving there are big discounts. Packt’s sale is still ongoing - $5 an ebook. Apress’s books are only $7 after Thanksgiving, I think both for ebooks and print.