another regretti i have is not looking into formats in which ebooks are available.
there is the .pdf file format. it has a layout that stays true to the way the person who saved the file made it, which can be nice to preserve the exact autism of the author or publisher but means the text can not re-flow, so when you try to read it on a little kobo, you can display it but it tries to display an entire page on a tiny screen, which is generally not usable.
there is the .epub file format which i think is free but it can include drm; not sure. the epub can re-flow, meaning it will look good even on a device with a small screen because the size of the letters can be chosen by the user. of course this means the intended autism-layout from the author or publisher is only respected in so far as to suggest a default font-style. i've seen little e-ink readers that are smaller then a smartphone even and with epub, a small device like that can be useful. it is easier to edit epub with a program like calibre but it is just a type of zip so you could just open it with a program that understands .zip and .rar files and remove/edit/add the textfiles, image-files and folders that make it up. I DIDN'T KNOW SOMETHING SO NICE EXISTED!
then there are other proprietary file formats like .mobi (that was bought by amazon) or .AZW, .AZW3, .KF8, .KFX (which were made by amazon. i stay away from those.