First, readers love series/sequels. However, the next book in any series should always be a standalone book, that is unless you really want to annoy your readers. And you should only write it because you have more to say, because you like the world in which you wrote it, or because you love the characters so much that you want to spend more time with them. Because if you did, your readers probably will, too.
If you’re going to write a sequel, though, don’t assume that a reader read book one. They may only have seen an intriguing cover or read the blurb for book two or three. I know of one writer who took a lot of backlash because her ‘sequel’ required readers to have read book one – they felt it was a cheat, that book two was just a way to make them buy book one. That’s why every book in the series should stand on its own.
You also have to be careful not to rehash all the info in book one in book two. It should be organic, as part of the story, something that can be shared via dialogue between two characters or as a memory. New readers need to know those shared experiences, so they understand what motivates the characters, why they’re friends or enemies. (Ditto Books Three, Four, and so on.) If you do your job right, then your readers will want to know more, and they’ll read Book One.
When you market Book Two, market it on its own merits. If you set it up correctly, the subtitle and cover will let readers know it’s part of a series. Series can be challenging for just that reason.
If you knew you were going to write a sequel to Book One, or a series, it’s much easier. Just make sure you actually have those books written. Or at least Book Two, with Book Three in-process, if it will be a series. There is nothing worse for a reader than having a writer talk about a sequel or series, but then they don’t deliver. If you didn’t, it gets complicated. For one thing, you need to join the books under a series name. Hopefully, you have an idea what that will be. It also means you’ll need to go back to book one and add that series name to the listing for the book. Ideally, too, the books should have similar covers. Those covers will automatically announce that there is a connection between the books. Without it, it makes it difficult for readers to know it’s a series. That may mean you need to purchase new covers, or contract with your original cover artist to produce a similar one. Similar covers will tell readers that it’s part of a sequel or series, reassuring them that they’ll find more books by their new favorite writer.
And nothing sells Book One like Book Two.