Julian Fellowes' "Downton Abbey: The Complete Scripts: Season One" (Morrow, $19.99) is not only a must-read, it's worth having just for the notes. Fellowes's notes cover everything from changes in the scripts for telecast to historical background to discussion of characters and more. His stage directions also complement the viewing. For example, here's his description Downton in the first episode:
April 1912 -- The sun is rising behind Downton Abbey, a great and splendid house in a great and splendid park. So secure does it appear, that it seems as if the way of life it represents will last for another thousand years. It won't.*
And here's where that asterisk takes you:
I was keen on Highclere to play Downton Abbey from the start, because it is an extraordinary expression of aristocratic confidence, a loud statment of the value of aristocracy. The house was built, or rather, adapted, in the 1830s, at the very beginning of Queen Victoria's reign, by Sir Charles Barry who was working on the Houses of Parliament at the time. Knowing as we did that the series, if it was going to run at all, would trace the decline of this particular class there seemed a nice irony in choosing a house that was so confident of their worth and value, and you get that from the first moment you arrive, when you enter the great atrium hall ...
Fellows goes into more detail in that note. And here's another example of his sense of detail:
The daughters are deliberately defined quite differently from the start. Mary is fairly hard, a bit snobbish and even selfish, but not essentially a bad person. .... Edith is not an originator and so she just goes along with what is happening. ... While the youngest, Sibyl, is essentially a rebel. ...The great thing about defining the characters means that you get a different mood out of all of them. Otherwise there is a danger that you have something generic called '"the daughters" and not much more.
In sum, you could enjoy the book even if you have not seen the series, so rich is the accompanying material.