The rules have applied to German cinemas showing adult films since 2002, but they are now being extended to cover books available on the internet.
Sex-themed e-books given curfew
Germans will be able to buy adult-themed e-books only between the hours of 22:00 and 06:00, under new rules.
It follows a complaint that a sexually explicit e-book titled Schlauchgeluste was too easy for children to obtain.
One book industry blogger said the decision, taken by the regulator Youth Protection Authority, was "just nuts".
The German Publishers and Booksellers Association confirmed that retailers were now considering ways to enforce the rules. It has been suggested that they will have to start tracking the titles that count as "youth endangering" under German law, isolate them in a specific section and make them invisible using filtering software.
But Jessica Sanger, from the legal team at the German Publishers and Booksellers Association, said: "A time lock doesn't seem to make a lot of sense to us as kids can find ways to circumvent that.
"A system that requires proof of age is harder to circumvent and that is the way we see things going."
Booksellers face fines of 50,000 euros (£35,000) if they do not comply with the rules.
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