Owner's Manual to be exact. You don't know how many times I look for an owner's manual in a customer's car and don't find one.
When I ask them where it is, I get a blank stare. Far be it they read the fucking thing when they buy a car. Maybe if they did, they wouldn't bother me for stupid shit all the time, like what the 'check engine' light means or the host of other multi-colored shit that goes on in the instrument panel.
You'd also know where your spare tire is and how to operate the jack. You'd also know how to open your hood, program your sound system, GPS, OnStar, compass, and a whole buncha other assorted shit you need to know about your car. Or you can pay me to hold your hand and walk you through it.
I could use the owner's manual in the car so I can decipher your fuse box (usually they're numbered and then I have to go and hunt for a cheat sheet to figure out which fuse controls what), figure out the grade and type of motor oil, transmission fluid, antifreeze, and a whole host of other fluids your manufacturer recommends. It's also easier for me to find the procedure to reset your 'oil change light' without having to search through my huge Service Bay Manual.
It also helps the tow truck driver if you're broke on the side of the road. Some cars have to be towed a specific way or it will do damage to the drivetrain, electronics, whatever.
Want to tow a trailer? Your manual tells you that too. It tells you how to load your car safely (think that doesn't matter? Put a few hundred pounds in the trunk and try and control the car at any type of speed). It also tells you about the tires (size, speed rating, and tread wear rating) and how much, or little, they can be inflated for safe operation.
Your owner's manual is the
most important tool a driver can have in the car with them. Read it (sit in the fucking car when you do and familiarize yourself with what they're talking about) and then leave it in the car. No, the two minutes your salesman spent going over shit with you doesn't count. If you don't have one, go to the dealer and order one. It's a small price to pay for a boatload of useful information.