I didn't say it's in some book, silly, I quoted someone who actually gave you the source, lmao.
The Imperial Star Destroyer has enough firepower to reduce a civilized world to slag
Imperial Sourcebook, p.61
These colossal, wedge-shaped behemoths, bristling with turboweapons and carrying entire TIE squadrons within them, each possess more firepower than the entire planetary forces of most worlds, and can reduce a planet surface to smoking debris in a matter of hours.
Star Wars Technical Journal, vol.2, p.17
... to rendezvous at Dankayo and reduce the tiny base to molten slag. Even before the last of its atmosphere drifted away, before the dense clouds of atomized topsoil could begin to settle, Imperial transports Elusive and Timely, as well as a complement of TIE fighters, moved in to perform "mop-up" operations and a through search of Dankayo's now evenly-cratered surface.
Scavenger Hunt, p.3
Transmitted from Dankayo to Alliance Com Buoy 965C shortly after the Imperial attack
Entry I
As instructed, I have remained behind until the last of our transports departed safely into hyperspace. Imperial Star Destroyers have so thoroughly blasted Dankayo that I fear for my safety, even in this deep-planet survival shelter.
Scavenger Hunt, p.20
"Sir, what about bombardment? Is there a stage for that?"
"Blasting a planet from orbit is easy — you don't need me to tell you how to do that. Limited orbital strikes would occur during the invasion stage. Just hope you are never given a Base Delta Zero order, lieutenant. Ah, yes, another question?"
"Sir, what's the Base Delta Zero order?"
"Base Delta Zero is the Imperial code order to destroy all population centres and resources, including industry, natural resources and cities. All other Imperial codes are subject to change, as you well know, but this code is always the same to prevent any confusion when the order is given. Base Delta Zero is rarely issued. ...."
A World to Conquor, Star Wars Adventure Journal #2, p.256
This is one of the more famous abilities in vs. debate history, I'm kinda surprised you didn't know about it.