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In theory, @LaurenIpsum's answer makes perfect sense. However, is we look at the market, we see this is not how things are done in practice. For example, J.K. Rowling planned from the start to wri...
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#2: Initial revision
In theory, @LaurenIpsum's answer makes perfect sense. However, is we look at the market, we see this is not how things are done in practice. For example, J.K. Rowling planned from the start to write seven _Harry Potter_ books, and certain plot points were planned from the start. For example, Voldemort was always going to be defeated in the seventh book, Snape was always going to be in love with Lily, etc. However, other elements of the story have not been planned, and there are some contradictions between early books and later books. For example, in _Philosopher's Stone_, Hagrid says Lily and James were "head boy and head girl". In _Order of the Phoenix_ we learn that it was Lupin who became prefect. (See whole list of similar mistakes [here](https://harrypotter.fandom.com/wiki/Mistakes_in_the_Harry_Potter_books).) G.R.R. Martin is of course still in the process of writing the sixth book of what was supposed to be the trilogy _A Song of Ice and Fire_. Had he done all the writing and editing from the start, he would have known this is not a trilogy. Or he would have cut a lot of material. At any rate, we would not have been waiting a decade for the next book. The reason that the market works this way is twofold, I believe. First, a published book feeds the author. And the author of course needs the money. So there's financial pressure to get the book out there and start getting paid for it. Second, if for some reason the first book fails, time invested in the sequels, that are now not going to be published, can appear "a waste". All that is not to say that @LaurenIpsum's logic is wrong. The books _should_ be interconnected, and it _is_ a problem when you'd like to change something, but it has already been published, so you can't. A way to mitigate that while still getting the first book of the series published ASAP would be to at least plan out the next volumes, have an outline, some key scenes that you see clearly. Those would ensure that you know where you're going, so you don't suddenly find yourself in the middle of the series, with no way to continue. And of course, there's no reason why you shouldn't start working on book 2 while querying agents with book 1.