Frederick John Corbitt worked under 4 British Monarchs.
His book was published in 1956 and the entire text
is available in the link.
Full text of "MY TWENTY YEARS IN BUCKINGHAM PALACE"
It is an easy and quick read. (Although a few typos.)
Interesting stories about George V & Queen Mary, George VI & Queen Elizabeth (QEQM), the current Queen & Prince Philip, and Princess Margaret & Peter Townsend.
Here is the list of chapters.
1 I Join the Royal Staff 3
2 Occasions of State 19
3 Three Royal Weddings 37
4 Catering for Celebrations 44
5 Racing at Cowes and Ascot 50
6 How Buckingham Palace Works 57
7 Balmoral Days and Tales 105
8 When Royalty Celebrates Christmases
and Birthdays 121
9 The King in War 145
10 Four Reigns 175
11 Edward the King 213
12 Another Royal Romance: Princess Margaret
and Peter Townsend 228
13 The Ever-Changing, Ever-Stable Monarchy 241