Although he is "best known" as the author of Modesty Blaise, I personally knew him best for that series of unusually strong romance novels. I don't think I would have ever guessed that they were written by a man; I liked them for their plucky and relatively believable heroines, romantic and adventurous plots, and for the glimpses they gave me of places like China, Venice and Cornwall, all of which I have now visited myself.
My favourite will always be Merlin's Keep, with its hint of the supernatural and extravagantly romantic (in every sense) plot. But I also remember Tregaron's Daughter and Stranger at Wildings very fondly. I've re-read all of them many times - they are the ultimate in comforting, escapist fun.
Reading the obit makes me think that perhaps I should try the Modesty Blaise books. But I already have to thank Mr. O'Donnell, Ms Brent, for hours of happy reading.