Demonstratives are a special group of adjectives/determiners (it's a debate in the linguistic community...), but adjectives/determiners nonetheless. In Turkish they behave exactly like adjectives :)
"Bu kitaplar" has two possibilities: 1. These books 2. These are books In order to avoid ambiguity we can use the sentence "bu kitaplardır" to exclusively convey the meaning 2. Still I believe that the translation 2 should be accepted in order not only to be fair, but also realistic.
You already identify it is plural by adding the plural suffix "-ler/-lar" on the "kitap". "Bunlar kitaplar iyi." is incorrect to the grammar rules of Turkish, it must be "Bu kitaplar iyi" (These books are good.). Thus you cannot add "-ler/-lar" on demonstrative pronouns, if you add on nouns.
- As a side note: "Bunlar kitaplar." means a different thing and it is "These are (the) books.".