This one can be time-consuming (and let's face it, who actually reads that tiny print on the pages of mini-books?) but if you do want to make your mini-books readable, you need clear print.
Sometimes you can find actual images of pages online (my go-to site is Project Gutenberg); I found pages for Humpty Dumpty, Raggedy Ann, Pride and Prejudice and Kate Greenaway that I could paste into my template. If it's a longer book, you'd need to just choose some of the pages, or your mini book would be too thick to manage.
However, sometimes you might want to make your own pages with text typed in. I've done this by typing (or copying/pasting) text into text boxes in a Word document, re-sizing the text to fit my 'pages' and using a suitable font to match the style of the book. I find 3 or 4 text boxes across a Word doc are a good size; still big enough to manage, but not too big to use for screenshots.
You can also paste pictures into the text boxes (or paste onto your document, make them 'in front of text' and move them where you want them). You can add illustrations, a cover, frontispiece text/image this way, and also add in 'endpaper' patterns if you like.
I find the hardest part is choosing and formatting the text. You can just copy the text from first part of the book, selecting the right amount to fit into each text box 'page'; I did this with the first few chapters of 'Wizard of Oz.
Or you might want to do your own precis of the story. I wanted more 'picture' pages for Grimm's Fairy Tales than the original book has, so I used the first bit of some of the stories, then did a short precis of the rest. I also found a lovely old print image from Rawpixel for the 'endpaper' page.
I like to use about 12 pages (including 'endpaper' pages and title page), as that makes the book thick enough to look like a book, but not too thick to get all the pages lined up and to fit inside the cover. If you want more pages, you'll need to make your book cover spine a bit wider.
When you're happy with your pages, can take screenshots of each line of pages, paste them into a new Word doc, re-size them (and make 'in front of text') then paste them into the normal template, to fit about 8 mini pages across your doc page, ready to print (as in the tutorial
here).
If you'd like a template of empty text boxes on a Word doc, here is one (with 4 text boxes across the page, narrow margins) you can download:
Of course, you can also create your own stories, poems etc. and add your own illustrations or photos to make a really individual min book!