kind, sort, and type. Though quite common in casual speech, constructions such as these kind of things, those type of people are ungrammatical and should be avoided in careful speech or writing. This sort of thing is a perfectly acceptable phrase; problems arise when a plural form is required because the singular form does not seem inclusive enough. These sorts of thing or those types of books are grammatically correct. Understood strictly, however, these phrases imply that there are several different categories of thing or book involved. Probably the best and most elegant way of overcoming the difficulty is to reverse the construction: books of that kind (many books, all of the same type) or books of those kinds (many books of several different types)