[Hellmark]

These aren't the only ones free for sony.

Paranoia by Joesph Finder
Serial by Jack Kilborn
Blood Engines by TA Pratt
Red Mars by Kim Stanley Robinson
Assassin's Apprentice by Robin Hobb
Star Wars: Lost Tribe of the Sith - Precipice by John Jackson Miller

(The following are romance books)
Darkfever by Karen Marie Moning
Kiss of Midnight by Lara Adrian
Kiss me Deadly by Michelle Hauf
Slow Hands by Leslie Kelly
His Lady Mistress by Elizabeth Rolls
Irresistible Forces by Brenda Jackson
Price of Passion by Susan Napier
The Bride's Baby by Liz Fielding
Snow Bound by Janice Kay Johnson

Maximum Ride, His Majesty's Dragon, and Lost Tribe of the Sith: Skyborn are also still free.