Tony Stark/Iron Man
Any Rick Riordan book. Both of you adore Rick's writing style, and Tony loves Leo Valdez from Heroes of Olympus. Both of you are utterly ashamed at the two Percy Jackson movies.
Pietro Maximoff/Quicksilver
The I Am Number Four books by Pittacus Lore. Pietro was the one to introduce the series to you, and you're working on reading I Am Number Four. Pietro loves how the Loriens like Four are speedy like him, and you're interested in the Legacies of Four and the others like him, but you're curious to how the Mogadorians killed One, Two, and Three. Pietro has promised to show you the I Am Number Four movie when you finally finish the book.
Thor
The Inheritance Cycle by Christopher Paolini and Magnus Chase and the Sword of Summer by Rick Riordan. Magnus Chase has obvious reasons - Norse mythology - and both of you love that fantasy-style aspects in the Inheritance Cycle. You love the way Rick Riordan combined Magnus Chase with the Kane Chronicles, Percy Jackson, and Heroes of Olympus.
Clint Barton/Hawkeye
The Hunger Games by Suzanne Collins. Need I say more? You're dating Hawkeye, the Avenger's archer. Of course he's in love with Katniss! You personally adore Finnick. Both of your hearts were broken many times during the series.
Bruce Banner/Hulk
The Maze Runner by James Dasher. A series not without heartbreaks, I mean, what series doesn't have heartbreaks these days, but still amazing. You both found yourself falling madly in love with Chuck, Newt, and Minho.
Steve Rogers/Captain America
Divergent by Veronica Roth. Ah, the heartbreak series of doom. Thanks for that Veronica. The love you and Steve had for Four? Yeah no. Not gonna end too great. You often found yourself wondering if Veronica Roth was devil spawn... then again, even if she was, you'd still read Divergent a thousand times.