It's really easy to see why this particular slave narrative was hugely popular in its day. The Craft couple's escape from the American South makes for a dramatic story. From a historical perspective, just realizing the sheer distance between Georgia and Philadelphia in the 19th century is quite interesting. It's not by any means the best written of slave narratives, but it's short and easy to read. Apart from that it is particularly interesting for these topics:
1. Life in the upper echelons of slavery in the American south as domestic servants and skilled workers, since this was the couple's background.
2. White slaves, wholly or mostly European, are a particular topic of interest. This was Ellen Craft's situation though others in her situation are mentioned.
3. The second escape to England, due to the Fugitive Slave Act, has some interesting insights into race relations in the free north.
4. Perceptions of slavery among slave owners in the American South. Ellen Craft's short period of masquerading as one of them provided the opportunity for her to hear and present these stories.