Adjectives are invariable. The moment you make it plural, it's no longer an adjective. It becomes a pronoun, and the sentence will be understood as: These are books.
Is this true for all adjectives, or just the "this, that, these, those" group?
Ektoraskan seems to state that bu is an adjective, and that bunlar is a pronoun. Surely bu is a pronoun.
"bu" can be a pronoun as well, sure :)
"this book," in this case, it is an adjective.
"This is a book," here is it not.
If "this/these" comes direct before a noun or an adjective modifying a noun, it is also an adjective. If it is by itself or is followed by "one," it is acting as a pronoun.
Ektoraskan says that when bu becomes plural, it is no longer an adjective, it is now a pronoun - thus bunlar kitaplar means these are books. AlexinNotTurkey says if 'these' comes direct before a noun, it is an adjective. In bunlar kitaplar, bunlar is direct before a noun, and is therefore an adjective. It has previously argued that 'bu kitaplar' means these books, while bunlar kitaplar means these are books. There is a contradiction here!
- Bu kitap = This book (Bu = adjective)
- Bu, kitap. = This is a book. (Bu = pronoun)
- Bu kitaplar = These books (Bu = adjective)
- Bunlar kitap = These (ones) are books (Bunlar = pronoun)