Johnny Depp and Amber Heard are obsessed with erotic literature.

The 50-year-old actor and his 28-year-old fiancee reportedly have a ''steamy'' relationship and have added spice to their intimate life with vintage sex tomes and photographic collections.

A source told ''They have added erotic literature that's even more explicit than '50 Shades of Grey' to their repertoire.

''Amber introduced Johnny to some racy vintage paperbacks and now he's into collecting old photographic sex books and footage from the 1950s and earlier.

''They'll read passages to each other from the books to spice up their already-hot relationship.''

The insider claims the couple - who got engaged on Christmas Eve (24.12.13) - regularly go through different fads in the bedroom.

The source said: ''It's pretty weird, but they've got a new thing every now and then -- and this is this month's fun thing they do together.''

The 'Transcendence' actor - who separated from long-term partner Vanessa Paradis, the mother of his children Lily-Rose, 14, and Jack, 11, in the summer of 2012 - recently insisted the age gap between himself and Amber is not a problem and claimed he has learned a lot from his younger lover.

Asked whether the age difference is ever a problem, he replied: ''I don't think so. I've got a lot of life experience, certainly, but I don't feel like I'm looking down on her. Or being her mentor. I actually feel like she's the one inspiring me!''