"He writes her a book."
Translation:Él le escribe un libro a ella.
In "He writes her a book, the word "writes" is the verb and its direct object is "book". If the direct object were "her" then we would say "He writes her", meaning he writes the word "her". By using "le" we clarify that "her" (or any singular recipient) is an indirect object. The sentence later identifies its indirect object. This is one of the more difficult challenges with Spanish, and I'm helped by breaking the sentence into two halves. Another example of two halves is "I see myself", which is "Me veo a mi mismo". This distinguishes the seer in the first half from the seer's reflection in the second half. It's kind of beautiful, imho.