What kind? It's hard to say so accurately. It depends on the situation and your friend's reality.
If your friend is a literary lover and usually pays attention to it, you can send some rare editions of famous books, or spend some time looking for books that are no longer printed, or books that have been limited edition.
If it is a female friend, it should be divided into several situations. If a female friend is not a young woman in literature and art, not a quiet temperament, she can send some best-selling novels, or some novels that are more ornamental and interesting, or some romantic comic books. If you are a young woman in literature and art, you can consider sending some poems or essays, as well as some comic books that are very warm and healing, which are very suitable for quiet girls.
If it is a male friend, it depends on his preference. Boys generally don't send novels. Especially the best-selling romance novels, boys should not like to read them. But mystery novels, science fiction and martial arts novels can be considered. If you are a sports enthusiast, you can send some books on sports, sports and fitness. If you are an anime fan, comics are definitely the most suitable, such as Huo Ying and One Piece.
It can also be used for different purposes. For example, a friend is taking an exam recently, or you need some professional books such as computer and finance for your daily work. You can send them as needed. If a friend has just moved into a new home recently, he can buy some books that look particularly tall and beautiful, which can play a decorative role. If a friend is depressed recently and wants to cheer him up by reading, you can send some inspirational books. If friends like to travel, they can send some travel notes or hand-painted atlases. If a friend has just added a child, you can send some books on parenting. If friends like cooking, they can buy some cookbooks. If a friend's family has children at school, they can also buy some famous books or popular science books according to their age.
Also, if you are a friend majoring in language, you can send a dictionary with good printing quality and beautiful appearance. In other words, I wanted a genuine Oxford dictionary, but later I succumbed to the arrogance of my roommates and bought a pirated one with them. ...
Because you really have too many possibilities for this problem, and there are many ifs ... you can choose according to your own needs. But no matter what books you send, you should find some long and good-looking books, especially girls'. Of course, knowledge itself is connotation, which can't be measured by appearance, but people who really like books definitely want their books to be valuable, good-looking and tall.