This is one of those books that people should take some time to read solely for it's historical significance, since it truly is a touchstone in the development of the novel as a distinct literary form. Released in 1740, it created a tidal wave of what we would now characterize as "media attention" and "popularity." Pamela was the right book at the right time and this confluence of time/place/text adds importance to the book itself.
The author, Samuel Richardson, was a commoner, without the aristocratic background of his rival, Henry Fielding or contemporary Tobias Smolett:
UNLIKE his great contemporary and rival, Henry Fielding, Samuel Richardson could boast of no connection, however remote, with an aristocratichouse. He himself has informed us that he came of a family " of middling note," in the county of Surrey, from which we may conjecture that his ancestors were small landed gentry or respectable yeomen. (<a href="[...]">Samuel Richardson By Clara Linklater Thomson</a>)
Thomson's biography mentions that in the 1740's, people were still a tad fuzzy on the concept of a fictional story, "Richardson was at once overwhelmed with letters from eager readers who longed to know whether the story was true." (Thomson, Samuel Richardson) It is against this back drop that you need to consider the development of the english novel as a real step forward in terms of the cultural sophistication of the readers. You can literally see the human mind moving away from the simplicity of the middle ages (and its literary forms.)
I think it's fair to say that the contribution of Pamela, in a nut shell, is the depth of psychological complexity of the characters. That is what the novel is all about: adding psychological depth to the depiction of character.
And so it is that the reader finds himself/herself relating to these characters, written three hundred plus years ago. Pamela tells the story of Pamela Edwards, a serving girl of 16. Her mistress dies and his son takes over the estate. The son has a thing for Pamela, so after she rejects a couple clumsy advances, he does what any 18th century nobleman would do: Has her kidnapped and imprisoned at his remote estate.
Now, anyone reading the above will understand that the activities depicted aren't in any way contemporary, but the depiction of character is. What we are witnessing in Pamela is the birth of literary consciousness of self and identity. It's interesting to read about but at the same time at 500 pages Pamela turns into a slog at time. You can see where it is an EARLY version of the novel as literary form- sine there is a resolution/climax half way through the book, followed by 200 pages of material that would no doubt not reach print these days.