The Knowledge is an incredibly whiggish pean to appropriate technology, as viewed through the lens of the end of the world. Images of the apocalypse and what happens afterwards are commonplace today.

The legacy of civilization would decay at strongly varying rates. The electrical grid and water system would fail almost instantaneously, within hours. Fresh fruits and meats would rot in days, dry preserved items might last for months or years before rodents breached the packaging, while canned goods could endure for decades. The material world would see similar destruction. Gasoline turns to sludge within a matter of months. Unmaintained buildings would fall apart in years. Some material objects, like glass, aluminum, and some plastics, are basically indestructible on human timescales. Even steel would take a long time to rust. Depending on how many survivors there were, they could live on the corpse of civilization for quite a time.

But quite a time is not forever, and at some point, civilization needs a reboot. The question there is what can be regained quickly, and what has hard limits in terms of complexity and resources. On the upside, there's a lot of useful inventions that were discovered surprisingly late in history that don't require advanced materials.  For example, the horse collar didn't arrive until the 15th century, and makes carts and plows much more useful. A 19th century schooner is made out of the same wood and rope as a trireme, and is an infinitely more handy ship. For mechanical engineering, knowing the Bessemer process to make ample steel, and the importance of the lathe and milling machine for precision parts, can let a reboot civilization skip directly from the 2nd century to the 19th. Similarly, sulfuric acid is fundamental to almost every industrial process, and can be obtained from a reaction with pyrite rocks and chlorine gas, which can be gotten from electrolyzed brine. Radio and antiseptics are similar low hanging technological fruit. On the downside, automobiles require a lot of precision engineering and rubber (rough if you don't live in the tropics) and petroleum and semi-conductors might be permanently lost fields. 

This book is fun, but there a lot of "simply take {raw material} and cook it in a vessel to extract {useful substance}", without much analysis of exactly how easy it would be to manage this without waste products, or in an environment where both food and fuel are tightly constrained. Everything seems easy-peasy lemon squeezy, while overlooking the parts that might be difficult difficult lemon difficult. In terms of wasted opportunities, there's not a lot on the idea of what could be scavenged and repurposed, aside from the back half of automobiles become hackney carriages, which is the fun part of the wastelands.  And the book ends with a resounding defense of the scientific method and innovation, which is hard to square with the utility of "ancient" wisdom after the apocalypse. Two millennium of believing the infallibility of Aristotle did enough damage to progress; would new thinkers be able to escape the shadow of thinkers who were right about much more?