For example, I have a friend who has read (and adores) the Mortal Instruments series and has actually read the fifth book which i have not. I would recommend The Night Circus by Erin Morgenstern.
To my friend that that reads even more than I do and loves both fiction and not fiction and strange things i would recommend The House of Leaves by Mark Z. Danielewski, but mostly she recommends books to me.
To people who like reading the next big thing i have recently been recommending The Bone Season By Samantha Shannon and have done recently.
To I regularly recommend The Hitch Hiker's Guide to the Galaxy by Douglas Adams and the Discworld books by Terry Pratchett partly because i love them and partly because the humor in them eases people into the science fiction and fantasy genre.
I recommend Threshold by Sara Douglass to people who like maths and fantasy and Ellis Peters books to people who like crime fiction but don't like books set in the modern day.
The most common thing i recommend to people in general is simply to read, and being a teacher i do this a lot. Don't just read tumblr or twitter or facebook but an actual book. I don't care if it is an ebook or a physical book, a piece of classic literature or a pulp fiction novella or even a graphic novel but read something because in the words of the great George R.R. Martin "A reader lives a thousand lives before he dies, said Jojen. The man who never reads lives only one." And life is too short to live it only once if you ask me. That goes for puppies too.
