Wonderful Life is a description of the Burgess Shale fauna, from the early part of the Cambrian Explosion. Gould describes some of the bizarre creatures found in that period such as the 5-eyed Opabina (which, as he puts it, "might grace the set of a science-fiction film, if considerably enlarged beyond its actual length of 43-70 mm") and the mysterious, and suitably-named, Hallucigenia, which we aren't even sure which end is front and which end is back, or even which is the top and which is the bottom!
Gould does more than simply describe these creatures, though. He also describes the history of their interpretations, starting from their discoverer, Wolcott's, attempts to identify them as simply primitive representatives of current animals, and why he made that error. Further investigation has shown that many of them do not fit in any of the current phyla, and those that do often do not fit in any of the subphyla of said phyla. It is a fascinating story of scientific investigation that lead to this radical reinterpretation.
Gould argues that the Burgess Shale fauna demonstrates that major groups often have maximal diversity of forms early in their history, and then those are reduced to a smaller number, with further diversity increasing only within those narrow bounds. The explosion of phyla in the early Cambrian, followed by the reduction to only 30 or so for most of the post-Cambrian world, being a perfect example. Gould's argument is that the creatures who survive are not necessarily superior, but, to a considerable extent, luck. That, if some alien biologist had visited Earth 540 million years ago, they could not have predicted which groups would survive and which would die. There would be no way of predicting that the rare Pikaia would survive to give rise to the dominant large-animal group (the chordates - that is, us) while the rather common and elegant Marella would die out with no descendants, as did the giant (relative to its time - about 2-3 feet in length, at a time when most animals were no more than 3 inches or so) and fiercesome Anamolocaris.
He argues for a much greater role for chance and historical contingency than is commonly accepted (to some extent, his ideas have become more common since he wrote this book).
The book is written with a minimum of technical jargon and should be of interest to any intelligent layman with an interest in the issues covered.