Petri Liukkonen and Ari Pesonen's writers biography site has an interesting article on Stewart/Innes.
The Lilly Library, Indiana University, has a collection of J I M Stewart's papers. The description of the collection indicates that Stewart planned his novels and kept track of their structures.
Stewart's agent, the firm of AP Watt, has a photograph of and a brief note about Stewart. I am not clear whether AP Watt represented Stewart for his academic work under his real name, or just for his writing as Innes. Derek Johns discusses the history of AP Watt and the role of the agent in The Guardian.
I am also the web manager of the novelist Martin Corrick's official site. Martin Corrick's first novel, The Navigation Log, has been described as an "intriguing, impressive debut", "told in beautifully plain language" - see reviews on site.
I sometimes sell books on the book auction site Your Bookmark.