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Whether or not your sequel can be read as a standalone novel, in the years that pass between the publication of one book and the next, it is quite likely that your readers would have forgotten some...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/35679 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/35679 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Whether or not your sequel can be read as a standalone novel, in the years that pass between the publication of one book and the next, it is quite likely that your readers would have forgotten some things. You should offer them a reminder. That said, if you repeat everything, it might be boring for a reader who reads the whole series in sequel. (When J.K. Rowling was still writing _Harry Potter_, years would pass between books. Nowadays, they are all available to the reader.) **A few key things** seem to me like a good balance between repeating everything, and repeating nothing. Something you can do is weave some of the "reminders" into the dialogue, for example - remember how everyone comments on Harry Potter's green eyes? Or it can be woven into the action: Harry might see Ron's _red hair_ from afar. Such reminders feel more organic than a repeat of a description we're already familiar with, serving both those who remember, and those who might have forgotten, and giving enough information to those who have never read the previous book(s).