This is actually a quotation within the book. Richard A. Gaunt in his Sir Robert Peel - The Life and Legacy quotes an obituary from Blackwood's Edinburgh Magazine of September 1850:
The death of Sir Robert Peel was an event so sudden, so unexpected, and so distressing that it excited a universal feeling of sympathy in the British heart, and stilled for a season every voice but that of melancholy among the immense multitudes to whom his public career had made him known. It stifled, during the first paroxysm of grief, even the loud wail of national distress: it obliterated the deep lines of party distinction: it caused to be forgotten the most painful feelings of extinguished confidence. All classes hastened to pay tribute to the eminent statesman who lay extended on the bed of premature death ... But there is a time for all things. There is a time for sorrow, and there is a time for justice. There is a season for sympathy with the agonised hearts of mourning relaties, and there is a season for calm reflection on the acts of public men. Death at once renders them the province of history.As to how long the gap between the sympathy and the calm reflection ought to be is hard to judge. Indeed, we have no clear views on it and never have had. Much depends on the attitude one takes to the particular public figure.