Evaluate whether a self-selected book is a comfortable, just-right, or stretch read, and build a personal reading ladder mixing all three.
Independent reading builds stamina fastest when you are not always reading the hardest book you can find, and not always reading the easiest one either. A reading ladder mixes comfortable reads (easy, relaxing, high success), just-right reads (a healthy challenge, understandable with some effort), and stretch reads (harder than usual, chosen deliberately to grow) -- deliberately, rather than by accident.
A quick way to check fit before committing to a book: open to any full page and read it, holding up one finger for every word or idea you do not understand. Zero or one finger by the end of the page usually means a comfortable read. Two or three usually means just-right. Four or more usually means a stretch read -- not automatically a bad choice, but one to go into with realistic expectations, maybe paired with audio support or a slower pace.
Mastery looks like: They can use the finger-count method to categorize a book's fit and can name a specific comfortable, just-right, and stretch title for their own reading ladder.
Common stumbles: Treating "harder" as always better, leading to constant stretch reading that burns out motivation, or the reverse -- avoiding any challenge at all.