I'll go with On the Road by Jack Kerouac. Simple plot overview: 2 guys road tripping across America. Themes range all over the place, so finding a single book that is about a road trip and contains the same themes is tough. I'll list a few.
Into the Wild by John Krakauer. The book focuses on Chris McCandless's frequent road trips into highly isolated areas. Similar in theme to On the Road because it explores themes of isolation and dissatisfaction.
The Great Gatsby. Some might consider that a stretch, but jazz music is a big part of On the Road. It is in Gatsby as well. Friendship is also key to Gatsby as well as a focus on cultural status differences (East/West Egg and the Valley of Ashes).
Fear and Loathing in Las Vegas -- similar because of the emphasis placed on drug and alcohol usage.
Thematically, sadness is a big part of On the Road. Sal sees it everywhere. Similarly, Holden Caulfield from Catcher in the Rye feels depressed by just about everything.
If you are looking for a book similar in style then pick another Kerouac book. Stylistically, his books are all in the same vein. He was a pioneer in the "beat" literature movement, and a lot of his writing is stream of consciousness.