The forest-fallow system is inconsistent with household property of any given plot of land. Property rights have to be created only when the cultivation cycle is shorter, and the quality of each single piece of land begins to matter. However, Boserup is not nostalgic about primitive societies. She added the concept of economies of scale. Many technologies can be properly exploited only if the population is dense enough.
Population growth makes urban civilization possible.
The second book is highly interesting, and has many insightful passages. As you would expect from a seminal work, The Conditions of Agricultural Growth launched and refocused many modern debates. Let me give two examples. This view is still diffused, if no longer dominant. In fact, the most advanced areas of Europe reached Stage 4 while China was already at Stage 5.