Learn Farsi: کتاب بالای میز است.

Spatial prepositions like bâlâ (above/up) need the Ezafe to connect them to the noun that follows (the table). Normally, the Ezafe is just a short "e" sound, but because bâlâ ends in a long vowel, we add a "y" sound to make it flow better. So, it becomes bâlâ-ye.

Does bâlâ-ye mean "on" or "above"? Could I use ru-ye here?

bâlâ-ye generally means "above" or "over" (often implying it isn't physically touching the surface, like a light above a table), but it is sometimes used interchangeably with "on top of". If the book is resting flat on the table, ru-ye is actually more accurate and common, but bâlâ-ye is still perfectly understood.

There is no word for "the" in this sentence. How do we know it means "the book" and not "a book"?

Farsi doesn't have a separate word for "the". A plain noun like ketâb usually means "the book" when it is the subject of a sentence like this. If the speaker wanted to say "a book", they would add the suffix "-i" to make it ketâbi.

Why is ast (is) at the very end of the sentence?

This is a great reminder of Farsi's word order! In English, the verb usually goes in the middle (The book IS above the table). In Farsi, the main verb is almost always placed at the very end of the sentence. The structure here is literally "The book above the table is".