I agree with dskim that Grout's "The History of Western Music" is certainly the classic and is used extensively in musicology courses. Another history text geared more to the music appreciation or survey level is Machlis' "The Enjoyment of Music". While not nearly as detailed as Grout, it still presents an excellent history of periods, styles, composers, concurrent trends in the fine arts, etc. If you can find it, I would recommend reading Machlis first as your introduction to music history, followed by Grout which will demand more concentration.