Probably shorter than the others. The advice I have on this can be condensed down in a few fairly simple points.
1) Begin with dialogue, action, motion.
This is my advice for all fantasy books- hell, any story- but it’s especially important with middle books. Even if the author usually begins with a rousing battle scene or the characters sparring with each other, there’s a temptation in a middle book to look back and have the character meditate on the last battle and every movement the villain made and the look in his best friend’s eyes as he died. The idea is that one has to summarize the book for the sake of any reader who jumps in in the middle of the series.
I actually think this is less of a problem than many people believe. Apart from most readers’ care to make sure they don’t start in the middle of a series, fantasy readers will usually be practiced in the feats of memory necessary to make the leap from book. You can certainly remind them of what happened, but you don’t have to summarize everything all over again.
2) Weave in summary from the previous book amongst new events.
Even when the book has a few pages of action or dialogue, or even- gasp!- a whole chapter before plunging into the summary, the reminiscing tends to be in clumps. Pages and paragraphs pass in which the character does nothing but smoke his pipe and think about the last battle.
Wrong. Pace-slowing. Boring.
The middle books need to be books in themselves, not only receptacles of past or future events. (If you make them only those receptacles, you’re treating them like bastard children again). If you absolutely must have the characters remembering specific events for whatever reason, have those memories woven in among new events. So Darker-Than-You-Think Goldeneyes is rushing down the hill in a skirmish against the Light Lord’s soldiers, and he thinks about the resemblances it has to the battle he fought against the Light Lord at the end of the previous book. Then he meets them, and sees a man who makes him start, since he thinks that this is the Light Lord’s lieutenant reborn for a moment, even though he knows he killed the lieutenant. Then he kills the man, and realizes that he’s wearing the super-magical armor the Light Lord was bragging about in the next book.
Much, much better to do this than to have Darker-Than-You-Think Goldeneyes scowling thoughtfully into the distance while the last battle replays in perfect detail in his mind.
3) Reduce your dependence on exact repetition.
One phrase that makes an impression on a character and becomes a mantra can be repeated again and again without making me have a seizure (unless it’s an extraordinarily stupid phrase). Replaying every nuance of the conversation between character and villain, or starting the second book off with the last scene from the first one, is lazy and usually gratuitous. Most characters don’t have memories that good, for the first. For the second, it gives the impression that no time has passed at all between the end of the last book and the beginning of the next, and that’s very rare in fantasy. Days can pass, or months, or even years, in the case of epics.
Don’t remind your readers in the middle of telling them a story that you are in the middle of a story. This is another reason to avoid the omniscient voice in the beginning of a book, too. How can the character know the summarizing you’re doing? (It’s even worse in those cases where the author includes things the characters couldn’t possibly know, without explanation).
4) Reduce your dependence on dreams.
One common way of sneaking summary in there is to have a character dreaming or nightmaring about the bad experience he had with the villain in the last book.
How many vividly detailed dreams that recall every nuance of a previous confrontation and every exact emotion of it have you had?
There’s no reason at all to shatter the reality of a fantasy world with this kind of dream. Prophetic dreams are a little different, since the authors use them less often, and most have the wits to make them obscure when they do. I do think that a heavy dependence on prophetic dreams can be just as dangerous, but their narrative function is different, and fits in more smoothly than the happy little summarizing dream.
5) If you’re jumping years, don’t cover every detail of those years, just the important ones.
If your second fantasy book opens ten years after the first, it’s unlikely that the characters would still be reacting exclusively to what happened to them in the first book. On the other hand, if you want a continuing story, you will have to show how they connect.
The trick is to decide what details are important and include them, and only them. Some authors have a skewed definition of what’s important, of course, which leads to presenting intimate details of healing magic the character’s done during those years and entirely ignoring that little marriage-and-child thing. But most of the time, authors will be able to sift out the important details. What prevents them from presenting only these is the conviction that the audience will want to know everything about the character.
On this, trust me. It’s unlikely that anyone else will love your character as you do, or want to know every intimate sticky detail. Summarizing every intimate sticky detail abuses the poor middle book again.
6) Don’t use the convenient “nothing happens” gap if you do jump time.
There are fantasy series out there where the first book ends with a cliffhanger or something near it, and the next one picks up years later, and apparently the villain and the hero have been just sitting on their goddamn hands for all the years in between. The author wanted years in between. She didn’t want to think about the logical consequences of a years-long war.
Narrative time has its own rules, of course- for one thing, you can show events as following each other which really happen simultaneously or even in reverse chronological order- but at some point common sense has to intervene. If you end the first book with a war about to begin, or someone about to kill someone else, or two characters having had a tense, screaming confrontation, you owe it to your readers and your story to detail the consequences of that setup. Don’t skip over it and leave it in the muck, and don’t assume that the characters just decided to go off and sulk instead of fighting after that kind of thing.
Poor middle books. *cuddles them*