Fritz Leiber The Mind Spider (1961)
As I may have mentioned a while back, Fritz Leiber's Change War is a conflict the width of the universe, spanning the entire course of history with two equally matched forces continually mucking about with each other's past, changing the present and all that good stuff. It features in the 1958 novella The Big Time and a handful of short stories, so to describe it as a series is possibly extravagant. Some, although by no means all of those short stories appear in this collection as the flip side of an Ace Double - enough at least to suggest a common theme even allowing for the couple that don't quite fit, and that The Mind Spider itself is apparently a Cthulhu mythos tale - this information gleaned from Wikipedia rather than anything conspicuously tentacular that stood out whilst reading it.
Anyway, none of this really matters so much as the quality of the stories which, whilst lacking anything genuinely mind-blowing, is nevertheless high. Leiber seems adept at narratives which swerve wildly in unexpected and occasionally van Vogtian directions, although his syntax is less angular and therefore easier on the brain. The Haunted Future - one of the longest here - serves to illustrate this, retaining the unsettled flavour of the surreal incidents of its first few pages even after the red eyes glaring from the dark are explained. It's not his best stuff, but it's good enough to keep the reader picking away until there's nothing left in the box, if a confectionary analogy isn't stretching the point too far.