Book Zero?

So uh… I found nothing useful on the internet regarding what I have in mind, so I thought I might as well ask an entire site of writers.

I'm planning on writing an entire book series and bringing it to fruition. 10 main books and anywhere from 1-12 one-shot novellas/guides. However, I also am planning on writing Book 0 (at the beginning of the series) and Book 00 (at the end of the series).

A Book 0, as I'm defining it, is a book full of needed exposition to understand the full story and grant a lot of background information about the characters before starting the "main" series. However, I would also like to make this book optional for readers. They don't have to read it to understand what's going on, but I also want to give them a reward for reading it (like a bit of special lore or something).

Alternatively, I'm also planning on writing a Book 00 for the end of the series, to show where the characters end up and what the world is like at the end versus what it was like at the beginning. Though I'm not quite sold on if I should write this or not.

I was wondering if anyone else had experiences with something like a Book 0 or a Book 00. Have you seen/read a series with a Book 0 or Book 00? Are you planning on making a Book 0 or Book 00 like I am? Any advice or tips when regarding a Book 0 or Book 00?

Hmm…now thinking about it, I'm not quite sure I've seen anything like this. The closest thing I've seen to a Book 0 or 00 is from The Kane Series written by Rick Riordan. It has an optional side book that gives some background and description of the characters, items, and Egyptian mythology. It wasn't at all necessary to read, as I completely understood the series as it was, but it was still kind of nice because he added images of what his characters look like. I'm sure that he explained and described his characters somewhere in the series, I just lack the memory space to hold onto information like that.

Now that you have brought the idea of a Book 0 or 00 to my attention, it's very tempting to try and attempt something like that. I think that a Book 0 and 00 would be both, really fun to write and beneficial to the readers (if I ever do publish a book).

Currently, I am all out of suggestions on how to write one, since I am only just now really acknowledging the thought of it and I have never tried to write one myself. Good luck!