Hamlin y White (4a Calle Oriente #12A, tel. 7832-7075, 9 a.m.–6:30 p.m. daily) has a good selection of books and international magazines.
Under the same ownership is Tiempo Libre (5a Avenida Norte #25, tel. 7832-1816, 9 a.m.–7 p.m. daily), with a wider assortment of books in English and Spanish, including Moon Handbooks.
On the west side of the plaza in the Portal del Comercio, La Casa del Conde (5a Avenida Norte #4, tel. 7832-3322, 9 a.m.–7 p.m. Mon.–Sat., 10 a.m.–7 p.m. Sun.) sells an assortment of travel guides in addition to material specifically relating to Guatemala, Central America, and the Mayan world, mostly in English.
A few doors down in the same complex, Un Poco De Todo (tel. 7832-4676, 9 a.m.–1:30 p.m. and 3–6:30 p.m. daily) also sells books, though mostly in Spanish.