Hi Haru, FORMAL vs INFORMAL is often a matter of vocabulary, 'rarety' and sentence structure. To be more specific and in reference to your examples: 1. 'this kind of book' is more commonly used 'thus' 'a book of this kind' could sound more formal (unless you are pointing at it:-)... 2. the same goes for 'these kinds of books' As for 'a book of these kinds' as unusual (and out of context) as it may sound, it could have a meaning of sorts in a sentence such as: 3. 'I read a book of these kinds of short stories recently...' It does not sound formal (even limit English) but could be conversational like ^_^. Bye bye, for now