History
The book I used for years is
A History of Western Music - Grout Burkholder, Palisca, by now in its 10th edition.
From the 8th edition onwards, the book has been rewritten from a more contextual perspective. Many connections are made with general history, cultural history with timelines.
With a digital subscription of the KB National Library of the Netherlands (www.kb.nl) the Cambridge Histories Online are a great addition.
The same goes for the subject articles of Oxford Music Online (Grove).