un and algún

Question

hi, in the case of saying ´en algún libro´etc where we mean some book, a book non specifically, can we use ´un´ in place of ´algún´?

Answer

Hola Ashlyn
When we are talking about something in a more "indefinite" way, "algún" tends to be used more to create this "indefinite" sense, however, depending on the tone of the speaker "un" can also convey the same as "algún". For example if I say "I saw something written in a book "(some book that I don't remember exactly which one) you could say both:
He visto esto escrito en un libro. 
He visto esto escrito en algún libro.
If the speaker has a "doubtful" face when saying "en un libro" that will be interpreted as "a book that I can't remember", the same way as "algún". 
I hope it clarifies it. 
Saludos