For last minute "oh no I forgot to pack my power cord" needs, try Best Buy or the Apple Store at the CambridgeSide Galleria, or MicroCenter, a 15 minute walk down Magazine Street from the Central Square T stop. (For the handful of non-North Americans, if you forgot power cord adapters, you can probably buy them at the MIT Coop (downstairs in the housewares or electronics section), just outside the Kendall Square T stop.)
There are two very good computer/technical book stores in Kendall Square, Quantum Books and the MIT Coop. (The Coop has more math and science than Quantum.) And the MIT Press Bookstore carries all in-print MIT Press computer books (and other MIT Press books, and books by MIT authors, and some other books).