But they are using carbon that would be availabe now regardless of the form it takes for their book.
The servers, cable, keyboards desktops and the Power plants creating the energy to run your online service are all using carbon the planet had previously removed from circulation.
Releasing that previously stored carbon is the problem, not binding a book. While both use stored carbon one does so for the production of his book once a year and is somewhat offset by the plant that was used to create it, yours is ticking away 24/7 burning that coal or LNG regardless of whether you are using it or not.
I have a car that gets 3mpg. Any coal fired Tesla or hybrid owner who comes up to me and tell me I'm killing the planet gets a quick slap in the chops.
Mine is a 100% non fossil fuelled powered but it is still traditional internal combustion and it produces the same emissions, difference being mine was allocated by the planet for use today not something from a few millions back. Ask a hybrid/electric owner what was involved in creating their batteries and hear crickets, more embarrassingly ask them what created the power and its network of delivery.
How can it be zero sum if we are adding something to a formula that Earth removed from the equation billions of years back?