That doesn't mean everyone starts at the beginning. In fact, I'd gamble from other discussions I've seen that as many as a third of those reading this post don't mind reading a series out of order. Personally, that would drive me nuts--guess I have a little OCD hidden in me somewhere--but knowing that the books will be read out of order means a writer has to take into account that just because something was explained in a previous book, doesn't mean that all the readers will be familiar with that explanation. (Or even that they will remember with most books coming out nearly a year apart.) So some rehashing of information is necessary, both to inform new readers and to remind everyone else.
The question then, is how much?
I know I've personally read books where I wanted to toss the book across the room as I yelled "I know all that already! Get on with it!" I've also read books where I sat there going "Who is this guy? Did we meet him already?" So there is definitely a happy medium.
When working with back story, the general rule is: No large info dumps. Work the information into the narrative in small slices. While what happened in a previous book isn't the same kind of back story as what happened to the character at say, age fifteen, it still happened before the opening of the current book. Quick reminder explanations of who someone is or how something works the first time it is encountered typically works well. The biggest issues tend to arise when dealing with something that changed drastically in a previous book (most likely due to the plot of that book) and trying to decide how much about that change (and plot) needs to be laid out for the reader. Again, this is (typically attempted to be) done without interrupting the forward progression of the story.
But recently I've noticed something new occurring in books. I wouldn't say it's a new trend because I haven't seen it enough to call it a trend, but I've run across at least a handful of books over the last few years which have a type of foreword that's not quite a prologue and basically quickly (as in maybe a manuscript page or two) lays out some of the pertinent events which occurred in previous books in the serious. Kind of like on TV shows. You know: "Previously on Vampire Diaries" followed by a series of quick scenes which brings viewers up to date before dropping us into this weeks episodes. (Yes, I admit it, I'm totally addicted to that show--it's deliciously dark and twisty.)
Happy hump day everyone! Also, remember to check back tomorrow for a fabulous guest post from M. K. Hobson.