Love the Carnegie Library. Nothing beats being close to the Oakland location! CMU and Pitt students don't know how good they have it.
The internal layout of the building is confusing, but worth an afternoon of exploration. There are older rows and rows of stacks, where you can get lost for a while or spook yourself out by looking up and down past the gaps in the glass... yep, that's another floor of books above you and below you. The museum also connects to the Carnegie Museums, which is sort of weird because you have to pay to browse the museum but not to enter the library.
In the warm months, grab a book here and some Italian soda and enjoy it in the awkward green space outside the library, right in the middle of Oakland. If it's cold, as it always in Pittsburgh, stay in the cafe *in the library* (brilliant, right?) for a cup of hot chocolate.