Singularity Sky
Singularity Sky (ISBN 0441010725) is a hard science fiction novel by author Charles Stross.
The story features widely spread human societies dealing with the aftermath of an information singularity.
Out of the singularity arose a mysterious and immensely powerful transhuman entity known as the Eschaton, which for unknown reasons scattered the majority of humanity to a large number of distant planets during its initial formation. It is usually benign and uninvolved in human affairs but which strictly enforces a few rules on human civilization out of apparent self-interest. The first and most important commandment of the Eschaton is "thou shalt not globally violate causality"; the use of faster than light travel to reach any point in one's own relative past is strictly prohibited. Eschaton apparently makes use of time travel itself, and whenever a civilization attempts to break this rule Eschaton stops it the moment before it would have been accomplished (often with immense overkill, such as in at least one case inducing the civilization's star to supernova). Eschaton rarely communicates and does not provide warnings beyond merely making its laws known.
One of the human civilizations with significant influence is the Earth-based UN, a distant descendant of the modern-day United Nations. Its agents wield their influence to attempt to keep other civilizations from breaking Eschaton's rules out of humanitarian concern over the collateral damage that will result if Eschaton is pushed to enforce them itself. Unknown to human civilization at large, Eschaton has its own human agents that work to this end as well. The novel features the exploits of one such agent, Martin Springfield, an engineer specializing in faster-than-light starship engines. He is hired by the New Republic (a relatively backward neo-luddite civilization) to upgrade the faster than light engines of their fleet of warships, and it is strongly suspected that, not comprehending the capabilities of Eschaton, the New Republic intends to attempt an "end run" around the rule against tampering with causality.
This attempt comes to pass sooner than expected when the New Republic colony at Rochard's World is "attacked" by the Festival, a nomadic transhuman society that freely trades highly technologically advanced goods and services with individuals in exchange for personal stories and entertainment information. This is seen as an assault on the strictly controlled and low-tech New Republic way of life, and the fleet is dispatched with Martin Springfield and a UN ambassador named Rachel brought along.