"He is reading a book."
I thought that 在 was filling the role of the "present progressive".
Does 正 make the sentence more clear or natural sounding?
I am under the impression that Mandarin speakers omit as much as possible whenever context allows for the omission. Given that, unless 正 is mandatory or unless it provides a more natural sounding sentence, I'd imagine it would be avoided, and that it's addition is unnecessary.
Native or advanced speakers, please elaborate if possible.