A previous post discussed genres that are not saturated. I was thinking of one area that is difficult to find books in. Not a genre as such but in educational circles these books are called High Interest/Low Vocab. They are for low readers, for example, 12 and 13 year olds reading at a Grade 2 - 3 level. They don't want to read about a couple of kids making mud pies. They are usually quite turned off reading but in school we need to find books for them to read at their level. Characters their own age or slightly older having age typical adventures.
It's not just the vocabulary; it is also the size of the font, the spacing between lines, the length of the chapters, the inclusion of line drawings every 4 or 5 pages. We have to order them through specific educational publishers; I don't think mainstream publishers have them nor have I seen many indicated as such in book stores or on line. Two books come up on Amazon in a search for 'High interest Low readability books for teens.' It may be that the demand isn't great because these students often don't consider reading a leisure activity they want to engage in. At least that has been my experience but perhaps the right books might make a difference.
The graphic novel 'Bone' is popular with these students also because of the minimized amount of text.
I know I think about vocabulary even with books for adults. I read once that the average newspaper is at a Grade 6 reading level.