Agatha Christie is one of my favorite authors and I have almost all of her mysteries on the shelves in my home library. Most of the novels were purchased at library and other used-book sales.
I also have a couple of her earliest books on my Kindle. Neither “Secret Adversary” nor “The Mysterious Affair at Styles” cost me a penny to download.
That’s because you can find many electronic versions of older books available for free because their copyright has expired. That’s how I loaded my Kindle with classics like Jane Austen’s “Pride and Prejudice.”
Check out Project Gutenberg’s free e-books at www.gutenberg.org. The site says it offers more than 40,000 free e-books with expired copyrights and it has links to other sites.
Sellers like Amazon and Barnes & Noble also often have free digital versions of the classics.