Addenda:

*) If two events occur, the speed of the travelling Now is what keeps them apart. This is what stops everything happening at once, which would be most embarrassing, and lead to all sorts of conspiracy theories.

*) The relevant relativistic equation is:

dx² + dy² + dz² - c²dt² = 0

.... I *think*. Corrections humbly invited from any passing physicist.

--- Oh, and as it happens, the book in question was Thief of Time. This may or may not be totally coincidental.