Learn Spanish (Latin America): El principio del libro es lento.

It actually means both! In this sentence, el principio means "the beginning" or "the start" of the book. However, it is also the Spanish word for "principle" (a moral rule or fundamental truth). The context usually makes it obvious which one you need.

Why do we use es instead of está for "is slow" here?

We use es (from the verb ser) because the slow pacing is an inherent, permanent characteristic of the book's beginning. We use estar for temporary states or conditions. If your e-reader was lagging while opening the book, you might say the device está lento, but the story itself permanently es lento.

Why is it del libro and not de el libro?

As a quick refresher, whenever the preposition de (of) is followed directly by the masculine singular article el (the), they must combine to form the mandatory contraction del. You can never write de el unless El is part of a capitalized proper noun, like a city name or a specific title.