* The most prolific writer using this name was Mary Elizabeth Braddon. Braddon, a Victorian era author, used the name Bertha M. Clay to write a huge number of sentimental novels, often focusing on romance and melodrama. These books were popular in their day, and many are still available today.
* Other authors, particularly in the early 20th century, also used the name Bertha M. Clay. These writers continued to publish similar style books, though their work is less well-known than Braddon's.
Therefore, to find out which books were written by Bertha M. Clay, you need to specify which author you're interested in.
If you're interested in the work of Mary Elizabeth Braddon, you can find lists of her books online. You can also check your local library or used bookstore for her novels.
Let me know if you have any more questions.