Let’s start with the information you need most: Dougal Dixon’s speculative evolution classic The New Dinosaurs , which imagines the biota of today if the K-Pg extinction event had never happened, is being reprinted in a handsomely-produced new edition from Breakdown Press. Here’s the website, open for pre-orders (link); the book ships on August 11. Do yourself a favor and grab a copy of this absolute banger.
Matt Wedel (2025) studied this question.