The suggestion it gave me was “...the marked book.” I work in a context with used books and that made me picture one with markings all over the cover. The only other meaning of marked as an adjective that I can think of is “significant” as in a marked increase in some statistic. Definitely not “excellent” which it now shows here.
I guess marked and indicated are at least somewhat synonymous in this context.
I think because it makes it sound like there is only one excellent book, and he's reading it.
Does it also make it sound like there is only one boy?
I think the sentence sounds perfectly natural in English, if we assume a context in which a particular boy and a particular excellent book have already been identified.