This is perhaps one of the rarest of game products--something which truly brings something new to the table. In the more than 30 years I've been buying RPG products, I've seen the same ideas and themes recycled over and over again (not that there's necessarily anything wrong with that, as long as it's done well), and it's fairly rare these days for me to stumble upon something really unique. This product is--it's wonderfully quirky, and even if you don't want to make full use of it (which would radically change an existing campaign setting), there's certainly an idea or two worth borrowing which makes it worth the purchase price.
As the description reads, this product introduces a new set of rules that can be added to a DCC campaign in order to dramatically change the very notion of life and death. The author notes that the idea came about when a cleric in his Crawljammer campaign destroyed a precious artifact with the express purpose of tearing down the veil between life and death in accordance with a cryptic message from his deity. This took the whole table by surprise, and the author generated these rules to explain what happened next. In short, under these rules characters no longer really die as they normally would--you can track hit points if you wish, but they don't really mean anything anymore. Your character's soul can continue to occupy its body, its shell, as long as it is largely intact. Only severe damage (beheading, being cut in half, etc.) will force you to flee your ruined flesh.
Even then, you're not finished! Your soul can attempt to steal another body, probably by seizing it from its current owner, forcing this hapless individual to lie dormant in some remote corner of the brain while you pilot the body. But beware of attemting to possess a body which already has two souls like this; the presence of the third will force all three souls to merge into a psychotic killing machine!
Needless to say, this massive change also brings a number of additional alterations as well--critical hits become more deadly, spell effects change, etc. However, traditional undead still exist (if they were created before the Age of Undying is ushered in, for example), though new forms of "undying" golem-like monsters can be created via "Stitchermen," (which a PC Technomancer could become).
Finally, as noted above with the Technomancer comment, the author of this book also wrote the excellent Crawljammer magazines, so classes from that setting are supported as well by the new rules.
In short, this is a supplement loaded with creative and off-the-wall ideas that an enterprising game master can make good use of, though it will radically change your campaign! However, as the author notes, the Age of Undying could be reversed, so it could be used temporarily if one of your PCs does something drastic like shatter a priceless artifact...it's always good for a game master to have an idea like this handy when such recklessness occurs!