outside reading

[piousoul]

Students' books are usually categorized into textbooks, reference books, and those for outside reading.


Is it better to substitute outside readings for the words in bold?
Which is more colloquial for a native speaker?
Thanks.

[Mister Micawber]

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Either is fine; I sense no difference (but do you mean 'colloquial'?-- that is spoken or casual written English, but the rest of your sentence seems more formal). For the sake of parallelism, I would choose 'outside readings'.
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[piousoul]

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Either is fine; I sense no difference (but do you mean 'colloquial'?-- that is spoken or casual written English, but the rest of your sentence seems more formal). For the sake of parallelism, I would choose 'outside readings'.
.

Thanks, Mister Micawber, for the feedback. I guess I get the idea.