Whittaker and Watson is the informal name of a book formally titled A Course of Modern Analysis, written by E. T. Whittaker and G. N. Watson, first published by Cambridge University Press in 1902. (The first edition was Whittaker's alone; it was in later editions with Watson that this book is best known.)
The book is notable for being the standard reference and textbook for a generation of Cambridge mathematicians including Littlewood and G. H. Hardy. Its reach was much further; André Weil in his obituary of the French mathematician Jean Delsarte noted that Delsarte always had a copy on his desk.
Today, the book retains much of its original appeal. Some idiosyncratic but interesting problems from the salad days of the Cambridge Mathematical Tripos are to be found in the exercises. It is terse, yet readable by the motivated student. It conforms to high standards of mathematical rigour, while compressing much actual formulaic information also.