Reddit reviews György Ligeti: Music of the Imagination

I don't think you'll likely find a book just about micropolyphony. Although it is sort of a type of counterpoint, it's not representative of a style or movement of composing in the same way that renaissance or baroque counterpoint is, so there's not as extensive a practice of it to write about. There's Ligeti and his works, and a few other composers that have done some similar things.

There is an article about micropolyphony called 'Micropolyphony:Motivations and Justifications Behind a Concept
Introduced by György Ligeti' which is maybe worth a read.

Paul Griffiths's book on Ligeti has a discussion of Atmospheres and micropolyphony.

Richard Steinitz's book Gyorgy Ligeti: Music of the Imagination also has a discussion of micropolyphony.

The best thing to do is probably to try and get a look at the score to Atmospheres by Ligeti. You'll learn a lot by just perusing the score.

You could also look into the term 'heterophony', which is not a million miles away from micropolyphony.