Learn Spanish (Latin America): Esos libros en el estante son muy caros. - Those books on the shelf are very expensive.

The singular word for "that" (masculine) is ese. But to make it plural ("those"), it changes to esos. This is a common exception to keep in mind! It has to match the masculine plural noun libros. (Feminine words are more regular: esa simply becomes esas).

I thought en meant "in", but here it translates to "on". How do I know which one it is?

The Spanish preposition en is very versatile and can mean "in", "on", or "at". You rely on context to know the exact meaning. Since books are usually placed on top of a shelf (el estante), "on" is the best English translation here.

Why do we use son instead of están to talk about the price?

In Spanish, we use the verb ser (here as son) to talk about the inherent characteristics or qualities of a noun. Even though a price could technically change, being expensive (caro) is treated as a basic description of these books, so we use ser instead of estar.

Why does the word caros end in an "-s"?