In addition to the ten Sapper books, there are also a number of continuations written by Sapper's best friend, Gerard Fairlie, who carried the series on after he died. However, these are all out of print and seem to be very difficult to obtain. The only Fairlie Drummond novel that's easy to get is Captain Bulldog Drummond, which I have. But alas, I have none of the other Fairlie ones.
I can however boast a nice uniform set of the ten Sapper BD books, issued by House of Stratus. (This company seemed to go out of business for a while, but they're back in it now.) And so I have set about reading them all. So far, I have read the first four: Bulldog Drummond, The Black Gang, The Third Round and The Final Count. I am just about to embark upon the fifth: The Female of the Species.
I'd actually read Bulldog Drummond some 20 years earlier, but had completely forgotten it. Back then, it was a Puffin paperback I read. The House of Stratus edition I read about a month ago had an interesting introduction that said Sapper was influenced by the Scarlet Pimpernel series, and that he in turn influenced Ian Fleming, who was apparently quite an avid Bulldog Drummond fan.
However, although Bulldog Drummond is classified as a children's book, I'm not entirely sure how suitable it is for younger readers. There's a romance angle that's dealt with very tastefully (the emphasis is definitely on adventure), but more seriously, people die, and it's not just the bad guys who kill. Drummond racks up a fair few victims himself. In the subsequent books too, people die violent and sometimes rather gruesome deaths.
That aside however, they are thoroughly entertaining for the most part, with some quite hilarious dialogue and gripping suspense. Some of the action could even be described as Blytonian, with secret rooms, caves and suchlike being explored at different times. The first three books are narrated in the third person, while many subsequent ones are told in the first person by various friends and acquaintances of Drummond's.
One aspect that I do find a little disturbing is some of the racist and anti-Semitic language. Jewish people are never mentioned favourably, usually described as "nasty", "little" and sometimes "dirty". There was also a most cringeworthy passage in The Final Count where a character describes Russia as being "run by homicidal alien Jews". Ick!
This too perhaps makes it not overly suitable for children - at least, certainly not for younger ones. A certain N word rhyming with "digger" also crops up once or twice, although it's used in an almost neutral way. I don't know that it really took on pejorative connotations until about the middle of the 20th century.Of course, in the 1920s and 1930s, when these books were first published, such sentiments were quite widespread, so no one would probably have paid them too much mind. The anti-Communist rhetoric of the first two books struck more of a chord with me though. I found myself nodding in vigorous agreement at some of the denunciations of it, and also of revolutionaries in general. Anyway, if you can cope with the occasional spot of racism and anti-Semitism, not to mention some of the violence, the books are otherwise a really enjoyable, suspenseful and sometimes very funny read. And I do recommend them, notwithstanding some of their less salubrious aspects.
So, who else here is a fan of this classic series from between the world wars? Which BD book is your favourite? If you have any of Fairlie's continuations, how do they compare with Sapper's originals, for you? Where does it rank for you? I think for me, the violence and anti-Semitism makes the series rank somewhat below Blyton, Biggles, Just William and so on (even though they all have some politically incorrect moments), but the narrative style and some of the adventures had by Drummond and his pals still makes it highly enjoyable. If you haven't got any of the books, or are looking to add to your collection, do try the House of Stratus paperbacks, which are of very nice quality and readily available from Amazon UK.
, our enjoying the entertaining parts of the Bulldog Drummond books while gritting our pc teeth at the unacceptable ethno-religious slurs, so regretably characteristic of the era, does 
north of the Bombay Hills
Incidentally I came across some radio dramatizations of Bulldog Drummond on I-tunes recently. The plays are from the 1940s or something. I downloaded several, but the quality was rather poor. It sounded like it had been recorded from a radio broadcast (as opposed to the producers getting hold of the original recording that was relayed on air), and had loads of static. So I gave up on it. 
It was almost like I'd stepped back in time to the 1950s and bought it fresh off the shelf.
without jackets.