There are two questions that look quite different in English:
Whose is this book?
What is this book like?
However, they are quite similar in Croatian, since both use question-words that are really adjectives:
kakav (kakv-) what... like / what kind
The questions above look like this in Croatian:
Čija je ova knjiga? Whose is this book?
Kakva je ova knjiga? What is this book like?
Bear in mind that both words change in case, gender and number as any other adjectives: you can ask questions whose or what...like not only for subjects, but for any noun in a sentence:
Čiju knjigu čitaš? Whose book do you read?
You would answer with a possessive adjective, in the right case, matching the case of the question-word (obviously, the accusative case here), number (singular) and gender (feminine):
— Aninu. Ana's.
— Tvoju. Yours.
With kakav (kakv-), it's possible to ask two kinds of questions – about kind and impression.
To ask about impressions, use the verb biti (je² +) be. An answer is basically an adjective standing for a subjective experience:
Kakva je ta knjiga? What is that book like?
— Malo dosadna. A bit boring.
It's also possible to ask about kind – then the noun we're asking is usually placed immediately after the question-word:
Kakvu knjigu čitaš? What kind of book do you read?
— Ljubavnu. A romance book. (lit. 'Love.')
(Pay attention how both kakvu and ljubavnu are forms that again exactly match the gender and case of the noun you are asking about – here, feminine singular, the accusative case – as obvious from their endings!)
(Maybe you're puzzled what exactly ljubavni is: it's an adjective derived from ljubav f love. In Croatian, nouns and adjectives are two distinct groups of words. While in English you can simply say love story, you cannot so in Croatian: you have to use the associated adjective, similar as English person vs. personal. You can find more in 57 School Yard: Relational Adjectives.)
Finally, it's possible to ask what book. Pay attention, not simply what (answers could be a book, a magazine, newspapers etc.), but what book.
For questions of type what book, what car, Croatian uses a specific question-adjective:
koji (specific forms) what... / which
Its forms basically follow the same pattern as moj my – there are longer and shorter forms. Again, you can use them as you want:
(Forms for fem. gender are, as usual, plain adjective forms, they are listed here just for completeness sake.)
Some forms also have optional vowels at their end. There's absolutely no difference between kog, koga, kojeg and kojega. For instance:
Koju knjigu čitaš? What book do you read?
(you would answer with the title of the book)
Croatian makes no difference between such questions and which-questions (asking what item from a given selection). In fact, the word koji is often translated as which and I personally have problems when to use what and when which in English, since my native language has no such distinction!
Croatian has no pronoun matching the English one. You can answer such questions using the determining adjective only (in the right case, gender and number, of course):
Koju jabuku želiš? Which apple do you want?
— Zelenu. The green one.
Likewise, if the context is known, you can ask just:
Koju želiš? Which one do you want?