It is normal for a friend to send a book. If I send a book to his college classmate, who is still a girl and married, I will usually send him some books about how women maintain family stability or how to face the workplace at work, because girls need to maintain family stability and then face the work when they get married. I think when they send a book, they should give her what she needs, because these things will be encountered in her life, and books will generally provide some standard answers.
If I send a book to a male friend of mine, I usually send some financial books. Boys generally like to make more money and should be more interested in investing in stock funds. My husband also likes reading the stock market and funds, so I think they should use or read such books if they are interested. If this boy is studying computer programming, you can usually send him some books related to computer programming. To sum up, giving is what you want. You can't give books that girls like to boys, and you can't give books that boys like to girls. Just send relevant books according to their needs!