It was quite common for book publishers to utilize a standard set of drop cap letter images during the 1800s. This floral drop cap letter A image was included in a letter set used by the same publisher in several different books. The publisher was T. Nelson & Sons.
The entire set can be viewed at Alphabet Set.
This image is copyright free and in the public domain anywhere that extends copyrights 70 years after death or at least 120 years after publication when the original illustrator is unknown.