Actually, the worst thing is exactly the same, no matter if it is book 2 or book 3.
It's how to remind the reader of what went before.
Too little, and you have them mystified.
Too much, and you bore them to tears.
Too poorly executed and you end up with silly conversations or paragraphs of recap or masses of flash backs.
The toughest thing is always, always, always to get it just right.
In fact, I don't think you can. Why not? Because no two readers are the same. Someone has a photographic memory and doesn't need to be reminded at all. Same with the guy who just put down book 1 and immediately picks up book 2.
Other readers have lousy memories and don't remember anything, or they let a couple of years lapse before they get around to reading book 2.
How can you please everyone?
You just have to take a stab at it.
In the past the fashion often seemed to be to write a synopsis of the previous book(s) and include that at the beginning. No one seems to do that these days. I must admit I always found such synopses incomprehensible. I much prefer to have necessary info inserted discreetly as I read. Bits here and there.
And believe me that is tough to do.
More on this tomorrow.