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Assuming the cabinets are mounted properly on the studs, the cabinets should easily hold your books with no problem whatsoever.

No decent contractor would mount these off the studs, but if you want to be really sure, you can mark the studs above or below the cabinets with a studfinder and then verify whether the mounting screws at the back of the cabinet line up to them. They're usually 16 inches apart.

Look at the screw heads inside the cabinet. Are they drywall screws (black, flat head) or wood screws (stainless steel, maybe dipped in a color) or tapcons (blue)? Drywall screws are another hint that someone didn't know what they were doing/taking shortcuts, and don't have the shear strength of wood screws (for studs) or tapcons (for masonary). If I saw drywall screws I would take the books down.

I have my tv on similar kitchen cabinets, which hang on cabinet brackets screwed into the wall (similar to IKEA's setup, which uses Blum). I think it's OK if installed properly, but if you have heavy plates inside, and then 10 books, that's kind of pushing it.

I hope so, because they look great up there :)
Assuming they are solid backed cabinets and not a rail hung type of system (like IKEA uses), you could always add another few fasteners of the appropriate type for your wall construction.

If you are really worried, you could also install wall mounted bookshelves immeadiately above the cabinets. (I'm thinking something like this: http://www.ikea.com/us/en/catalog/products/46693309

From the ground, you would have a similar look, but the added load wouldn't be on the cabinets.

Just to clarify, the proper hardware to install cabinets are specific wood screws called "cabinet screws", usually 3 1/2" with no threading for about an 1 1/4" below the head. Only the bottom screws are subjected to shear forces. The top screws are the most important and are subject to both tension and shear.

If they're installed correctly (onto the studs with proper cabinet screws), your cabinets will EASILY hold your books and heavy plates, etc.

Not sure about the safety, and sorry to play mum, but won't the books get ruined up there? Cooking grease etc? Ok, I guess it's not going to splatter up that high!

I am anti-open shelves in many cases for the reason you cite, nadyamadrid, but I have my cookbooks in a similar spot, and they are so high up that grime has not been a problem. They've been up there 5 years, so I guess my cabinets are properly installed!

I weigh 190lbs. and once stood on top of my cabinets to paint. Not smart, but without incident.

Good quality cabinets installed properly should not be a problem - BUT after seeing cabinets that had been fine for years come down suddenly, I realized how dangerous that can be if it should happen while someone is in the kitchen.

What about earthquakes? You do live in California, and a mild earthquake may knock the books down.
I'm a recent SFer and am paranoid about everything in an earthquake (including a glass of water near my computer).

The tops of my cabinets get amazingly greasy and dirty, and I don't cook that often. This has been true my whole life, no matter where I've lived.

the only things I've ever put above cabinets are seldom-used baking pans, covered with dish towels.

I'd take the books down and wait about six months, then check to see if it gets greasy and gross up there. If not, then put the books back up.

I second cashba's earthqake comment. I don't want books raining down on me during a temblor.