I might want to get going on Book 2. But knowing how much to repeat from Book 1 is becoming a bit of a struggle.
Getting going on Book 2 is a fabulous strategy. When I was doing the Unpubbed contest circuit, I noticed that the authors who were entering two titles at the same time seemed to do much better... in that they retired much sooner from the lists, and I infer that they made sales.
How much to repeat... is an important balance when you've built an alien world, and yet every book in the series has to be a stand-alone.
When I was writing Insufficient Mating Material (sequel to Forced Mate), my editor Alicia Condon suggested that I ought to take J K Rowling as my role model as regards backstory telling.
If course, I was not going get the page count or the ink. So, I spent a delightful summer acquainting myself with Harry Potter, and trying to extrapolate proportions for "potted" versions of my own backstory. (Bad pun. Couldn't resist. Sorry!)
Here's my take: (Somewhat repetitive)
1. Break any rule of thumb rather than bore your reader.
2. Avoid info dumps at all costs. (Six lines of explanation is more than enough.)
3. On any given page, tell the reader only what she absolutely must know in order to understand the current action, or rules of your alien world.
4. Delay telling as much as you can of the back story.
5. Reunions of beloved characters from the previous book are fun for your established readers, but not so much for someone coming cold to Book 2, not having read Book 1, so any cameo appearances must be meaningful and advance the new story.
6. Use family trees, charts, maps with annotations as creative and visually different techniques for communicating backstory, who's who info etc.
7. Do not rely on being able to use footnotes. Some editors will be nervous about the possibility of the printer being unable to line them up.
8. "Dear Reader" letters in the Front Matter are a possibility, but frequently are skipped by the very reader you wanted to bring up to speed.
9. Prologues ought to be short, but can be very useful and entertaining. A great example would be the J K Rowling scene where the Minister of Magic is obliged to brief the British Prime Minister.
10. Consider putting a fresh spin on the backstory by having someone else relate it... I like to remember that "Summer Lovin'" duet from Grease where the Olivia Newton-John and John Travolta characters gave different accounts of a sweet summer romance.
11. My personal favorite backstory comunicator is my own Grievous. A "Greek Chorus" character is extremely useful. Or an employee who habitually covers his backside by making absolutely sure he understands his orders.
What have I missed?