As a self-publisher, I am required to comply with this regulation. Unlike most other products, books in Germany cannot be sold at individual prices. Instead, publishers set a fixed retail price that everyone must adhere to — from large online retailers to small bookstores.
Specifically, this means: I cannot offer quantity discounts when you purchase multiple copies, include promotional gifts with the book, or issue vouchers. Even indirect benefits like "Buy the book and get a discount on another product" are not permitted.
The German Publishers and Booksellers Association monitors compliance with these rules and can impose warnings and fines of up to 10,000 euros for violations. This was made clear to me in a rather sternly worded letter regarding my first photo book. Exceptions only exist for books with obvious defects or when a title is older than 18 months and the publisher actively lifts the price restriction.
In theory, fixed book pricing is meant to protect diversity in the book market and prevent large retailers from pushing out smaller ones with dumping prices. Whether this system is still appropriate in the age of online commerce is certainly debatable. However, as a self-publisher, I must comply with these requirements — even though brick-and-mortar bookstores offer little support to independent publishers like myself.
Interesting to note: Fixed pricing in Germany is a peculiarity that, besides books, only exists for prescription medications and tobacco products. For medications, it serves to ensure nationwide supply to the population, while for tobacco products, it primarily helps the state control consumption.