A、Books that are easy to carry and easy to find. B、Books that are published recently. C、Books that are informative and easy to u

　　(16) There are several ways you can find out about the countries and places you wish to visit. You can talk to friends who have traveled to the places, or you can go and see a film about them, or you can read travel books. It seems that there are three kinds of travel books. The first is those books that give a personal, subjective account of travels which the author has actually made himself. If they are informative and have a good index, they can be useful to you when you are planning your travels. (17) The second kind is those books whose purpose is to give a purely objective description of things to be done and seen. If a well-read, cultured person has written such a book, then it is even more useful. It can be classified as a selective guide book. The third kind is those books which are called "a guide" to some place or other. If they are good, they will, in addition to their factual information, give an analysis or an interpretation. Like the first kind, they can be inspiring and entertaining. But their primary function is to assist the reader who wishes to plan in the most practical way. Whatever kind of travel book you choose, you must make sure that it does not describe everything as "marvelous," "fabulous" or "magical." (18) You must also note its date of publication because travel is a very practical affair and many things change quickly in the 21st century. Finally, you should make sure that the contents are well presented and easy to find.
　　16. What does the speaker mainly talk about?
　　17. What do we learn about the second kind of travel books?
　　18. What kind of books should be chosen according to the speaker?