All of the books on this list are sought after by tens of thousands (if not hundreds of thousands) of searchers. The top 10 list includes a broad selection of titles, including 2 Stephen King novels, a Nan Roberts novel, the Johnny Cash bio, and even Madonna's Sex book (still number one after 5 years).
But as diverse as this list might be, all of the titles on it have one thing in common: people want to buy them. Now, I know that rights issues are likely the primary reason you cannot buy most of these books as ebooks. (And while Madonna's Sex book would work as an iPad app, I seriously doubt it would get past Apple's censors.) But this list still stands as one of the easiest forms of market research.
Any title in the top 10 represents at a minimum tens of thousands of lost ebook sales over the past year, and that is money that publishers have left on the table. The same goes for books further down the list; each is an excellent opportunity to turn used book sales into new ebook sales.
Take Vincent Price's cookbook, for example. The star of horror films was also an accomplished chef and even had his own cooking show in the UK called Cooking Price-Wise. One of his cookbooks are still popular over 20 years after his death.
If someone took his cookbooks (he actually had several) and combined them with clips from his show the result would make a decent iPad app. People would buy it. Again, I'm sure that the reason no one has released said app is that they cannot get the rights to the books or the TV show, but this is still money left on the table.
And from what I can tell, the other reason some of these titles aren't available as ebooks is author or estate intransigence. Stephen king probably doesn't want his 2 works to be published again, and the same goes for Nora Roberts. But as for the CS Lewis or Madeline L'Engle titles, I don't know why the estate hasn't pushed to release the ebooks. They're not categorically opposed to the idea, so perhaps they don't see it as worth the effort.
If you're an author or publisher I would strongly urge you to look at this list and see what opportunities you might be missing.