(The links contain the spoilers.)
The Strand Magazine has a lovely piece on the detective of Margery Allingham, who is one of my very favorites of the gentlemen detectives.
The Great Detectives: Albert Campion is full of spoilers. Do not read it unless you've already read the series. I enjoyed the piece a lot, but again, I have read all the books many times over.
You could read the first six paragraphs of the essay and not be terribly spoiled -- possibly teased into reading the series that way.
Or into finding the BBC series Campion; it had two seasons and starred Peter Davison of Doctor Who fame. That's how I discovered Campion myself. :) Huzzah for Mystery! on PBS.
I did not know that walking tours of detectives existed. In the UK and the US only, sorry. Sherlock Holmes, Campion, Peter Wimsey -- and two tours for Agatha Christie -- before turning to NYC for Dashiell Hammett and Edgar Allen Poe.
If you don't know about the authors or their detectives, the tour guides will spoil their mysteries for you. They're for the dedicated enthusiasts.