I guess what I wonder is what you mean by readers? Are you talking books for children who are still not proficient readers, meaning somewhere before they could tackle a chapter book like Charlotte’s Web? Or do you simply mean books for each child to read for ‘literature’ on their own no matter their age?
My answer is different for each. For those still working on becoming proficient in the reading department we start with Now I’m Reading sets by Nora Gaydos. Then we move them up to the hardbound readers from All About Spelling/All About Reading (which I’ve aquired over the years when they first came out at a discount). I also have a few of the Pathway Readers which are hardbound and come in every reading level from very beginning to much older. I have a few elementary level ones but have not felt the need to get a full set. Once my kids can read a chapter book easily they don’t need these.
Now, my oldest only had the Now I’m Reading books, then we just worked slowly on slightly harder picture books until she was ready for chapter books around 6.5-7.
My 7 and 8 year olds just this week jumped from readers to trying a Magic Tree House chapter book on their own and are doing well. Not the highest quality of literature, but they fit the easy chapter book level and the kids love the history slant.
My 5 year old son taught himself to read fluently at age 3. He can read from the scriptures with few stumbles, and tackles chapter books at will. Had very little to do with me and my teaching!
So my recommendation for the learning to read fluent stage is to invest in a series or two of good readers like Now I’m Reading, All About Reading readers, and Pathway Readers. Each child can use these when they are ready and when they hit fluency they jump to good chapter books.
For all those fluently reading children we invest in a few new-to-us books per child per year (I’ve got baby #8 coming this summer). In this way we’ve build a home library the younger children benefit from, so I am beginning to find I don’t have any I really want to buy for the younger end of chapter books. We have a lot of quality picture books too. My kids think it is a treat to go to Half Price Books to each choose a title (which we do about twice a year).
And we’re best friends with the library and interlibrary loan. We go every Saturday just about and return books as needed (our library has a drive thru to pick up and return books even, so we can do it on the way to errands!).
Good luck! We also love Kindles and have a large library on there, from free through Project Gutenberg to Yesterday’s Classics and some of the Heritage History sets. What we have on one Kindle can be put on up to 6 devices.