The boy always forgets his book
"The boy always forgets his book" was translated by someone as "El chico siempre se le olvida de su libro." I thought it would be "El chico siempre olvida su libro." I can wrap my mind around the reflexive usage, but why "le"? If anything, I'd use "lo." But perhaps this is a leísmo?
3 Answers
There are various ways to use this verb:
El chico siempre olvida su libro.
El chico siempre olvida de su libro.
El chico siempre se olvida (de) su libro.
Al chico siempre se le olvida su libro. (This is the usage that puts responsibility of loss on the book).
In the translation given to you, le is not necessary and sounds awkward.
El chico siempre se le olvida de su libro.
I am not a native speaker but when I see this structure my head thinks along the lines of
"The boy's book always seems to get lost".
It removes the responsibility of forgetting it.
I may well be wrong and it does not answer your question. 
I think you can see better the distinction Julian is making if you use Yo instead of the third person, as it makes more clear what the verbs and pronouns refer to- so here is how I think of it:
Siempre se me olvida el libro
Sort of (interpreted liberally and only a way to think of it) The book always forgets itself on me. Verb to the book (book is subject, and "reflexive"), on me (I am the indirect object).
Siempre me olvido del libro
Again sort of- I always forget (on myself)/ it slips my mind about the book. Verb to me, on me ("reflexive")- book is direct object.
Siempre olvido el libro.
I always forget the book (I forget- I admit it). Verb to me, book is direct object.
