The Stolen Throne and The Calling give you some background info before origins and delves a little bit deeper into Loghain's and Marric's story (in the stolen throne) and Duncan's story (in The Calling).
Asunder is the 1st of 3 books that will tie into inquisition, starting during or right after the events in act 3 of da2. Gives good insight to the mage vs templar war, and how it truly got started. Also a little about our live in scarecrow Cole. 1st to bring up the civil war in Orlais, which was not mentioned in da2 (and it was war with ferelden that was brought up with allistair if he was king in da2, not the civil war)
The Masked Empire is the 2nd of 3 books that will tie into inquisition, right at the same time Asunder occurs. Gives insight on how the civil war in orlais started and hints to possible factions we may need to recruit in inquisition.
Last Flight is supposedly the 3rd of 3 books tying into inquisition, presumably at the same time as the other 2. We know its about a young female warden researching the history of the blights and she discovers a horrible truth about the wardens. It is also assumed that there will be a lot of flashbacks in this book as she is reading the notes of another time period. I believe this will give us insight as to what the wardens are currently up to, as was hinted by many of the writers in interviews. Last Flight doesnt hit shelves till August.
You dont have to read the books, the games do a good enough job of not leaving you out of the loop (unlike the witcher series). I highly recommend the books though, as they give a lot more in depth analysis to some characters we see whom we might not get to know as well in the games