Eternal Rider is a fantastic read that left me needing to read book two (Immortal Rider). I'll start by letting you all know that you do not have to read the Demonica series to understand what is going on. It might be helpful, but I had no issues and this is my first time reading a Larissa Ione book.
The series revolves around the four horsemen, Ares (War), Limos (Famine), Thanatos (Death), and Reseph (Pestilence). Eternal Rider focuses on Ares who is trying his hardest not to let his seal break and start the Apocalypse.
The story is told in Cara's (Ares' potential love interest) point of view. Cara saves a hellhound and finds herself surrounded by things she never knew existed. Ares comes to her rescue and you can see the sparks fly. Ares is written to be a gorgeous hunk of man, but you can tell he isn't used to having someone to care for much less someone caring about him. This leads to an interesting relationship where you can see the feelings but they aren't spoken until necessary.
I don't want to give too many spoilers, so I'll keep the review short. All you need to know is the book was excellent and if you enjoy paranormal romances at all this is a must read.