My real house is filled with bookcases, so I bought the tall, skinny birch wood bookcase for my little people as soon as I could, even though it didn't actually add any functionality to their house.
Birch doesn't go with the other office furniture, though, and I'm not all that fond of light wood, anyway, so I've recolored it to a darker color that matches the other furniture better.
I made this for my own use, but I'm sharing it here just in case anybody else wants it.
The necessary files are zipped; you'll need to unzip them before you install them. You can find installation instructions in the enclosed README file or in LadyCFII's useful post
here.
Edited to Add: Now that my family has reached generation 4, I've recolored the bookcase that becomes available in that generation, too; if you want that one, choose the file that's called
LargeDarkWoodBookcase.