The burning city

Blurb

The Burning City is a fantasy novel of social and political allegory by Larry Niven and Jerry Pournelle set in an analogue of Southern California in an imaginary past shortly after the sinking of Atlantis about 14,000 years ago in the twilight of a civilization then struggling and now vanished for lack of a crucial natural, and essentially non-renewable resource upon which almost all of its economy and technology depended. The vanishing resource is not oil but mana, something vital to the technology of magic and the metabolism of the supernatural. As mana becomes scarce gods sleep and finally die, unicorns get smaller and finally turn into hornless ponies, and magic becomes less and less effective and finally vanishes.
The book was published in 2000, and was followed by a sequel, Burning Tower in 2005. It is part of the same timeline as The Magic Goes Away