One of his own rules was that Rupert was never to be shown in a bathing costume. The little white hands are one thing, but what goes on underneath that red pullover and those yellow check trousers does not bear examination.
Algy Pug, Bill the Badger and Bingo the Brainy Pup are also better left in their bow-ties and Eton collars and jackets. Indeed, the old-fashioned outfits of all concerned have never changed over the years. Nor have they detracted a jot from the characters' lasting popularity.
According to Hilda Coe, who for many years wrote the verses for the Rupert stories, the reason Bestall thought they were so popular was ' because right always wins'. Indeed, his chief difficulty was how to keep Rupert 'doing right without making him a prig'.
But though Bestall's reputation rests entirely on Rupert, fewer than 60 pages out of 330 are devoted to the little bear.
Caroline G. Bott, who is Bestall's goddaughter, is keen to show us that there was a lot more to this modest and self-effacing man than might be realised.
Thus, in addition to many examples of his romantic and dreamy watercolours - including travel posters and magazine and book illustrations (Enid Blyton was a particularly well-pleased author) - there are lengthy extracts from his journals, with sketches, covering visits to Wales in 1912 and 1913 and a sea trip to Egypt and the Middle East with his parents in 1924.
The drawings speak louder than the words, though there are some enjoyable descriptions of places such as Cairo and one or two rather interesting asides during his travels.
Can it really be true, for example, that Lord Kitchener introduced kites (the birds, that is) into Egypt 'for scavenging purposes'?
There are also a couple of chapters on Bestall's early life, including touching letters from France where he served from 1916-18 in the 35th (Bantam) Division - initially formed to accommodate men below the general weight limit required for active service.
Mrs Bott has indeed mostly allowed others to tell the story of this kindly, generous-spirited and very talented artist.
Hers is a real labour of love. The 35-page index lists every single piece of artwork that Bestall ever produced, plus fee - including all the Rupert stories and 40 years' worth of origami figures which were included in many of the Rupert Annuals.
To all those who were brought up on Rupert - as well as to many with a hankering for a gentler age - this book will be an unexpected pleasure.