I'm currently working on a first person piece. There are things I think it is essential the reader knows, but that my main character wouldn't know or wouldn't find out until much later. What is the best way to write this?
My preference would be for them to be visibly missing things that the main character notices are missing, so that when the reader feels they are missing they know it's that way for a reason. Bits from other POVs; well, it depends on your story, but I'm inclined to think of that as not playing fair.
A lot also depends on your distance from the story. If you're in headlong as-it-happens first person, or even a direct present-tense braindump sort of first, you really don't have any space at all outside of the immediate moment; if you are writing it at the distance of a memoir or a document someone is leaving after the story is done, there's nothing to keep your narrator from interjecting with bits of information they did not have at that point in the story.
It
is possible to make things clear in the story to the reader without the narrator getting them, to some extent, but it is difficult. Most readers seem to assume that a first person narrator is reliable, which kind of bemuses me as a reaction. There are bits in the Dresden Files where things escape Harry at the time, and he doesn't figure them out until quite a bit later, but they are actually in from early on; Steven Brust does a
lot of this in the chronologically earlier Vlad Taltos books, though they do not really become that visible until much later in the series. There is an amazing section in Michael Moorcock's
The Laughter of Carthage, a memoir of the narrator crossing the US as a speaker for the Ku Klux Klan, in which it is clearly obvious
both that the narrator as a young man in the 1920s is completely wrong about who is his friend, who is his enemy, and what anyone's motivations are,
and that the narrator as an obnoxious old man in his seventies writing this down is still completely wrong about these things, though not in exactly the same way. I'd suggest these as examples if you're interested in looking at how this particular approach can be made to work.