The Houghton-Mifflin Readers

1. Level 1. Getting a head start
2. level 2. Getting ready to read
3. level 3. Tigers, Lions, Dinosaurs
4. level 4. Rainbows [Primer]
5. level 5. Signposts [grade 1]
6. level 6. Secrets [grade 2A]
7. level 7. Rewards [grade 2B]
8. level 8. Panorama [grade 3A]
9. level 9. Fiesta [grade 3B]
10. level 10. Kaleidoscope [grade 4]
11. level 11. Images [grade 5]
12. level 12. Galaxies [grade 6]
13. level 13. Serendipity [grade 7]
14. level 14. Diversity [grade 8]

Want to read them? You can, for free!
Archive.org has them!  https://archive.org/details/inlibrary
 Put the book's name into the search box in the upper right, then in the search results page, on the left, check the box for TEXT, then when that loads, check the years 1971 and 1974. And now you can see them, and actually read them clear through if you make a free account. Awesome!
There is even a cool nostalgia site about them, were I got most of the images from. https://hmr.posthaven.com/