I see now that some people call this a "future passive participle", but it is conventionally called a gerundive. So I wouldn't think of "going to be read" at all if I were to translate it.
A sense of prediction or obligation is inherent in any gerundive. The most literal translation is as follows—by most literal I mean the one that works in most situations, especially in a neutral context, and that is the least dependent on context in general:
Puer librum legendum habet. — "The boy has a book to read."
In English, to + infinitive can express that something will happen or should happen. Although this English construction and the gerundive are not related, they both happen to reflect this double modality: probability (will happen) and desirability (should happen). Necessity can be a kind of desirability ("this room needs to be cleaned") or a kind of probability ("random DNA mutations are necessary for cancer to develop"). I beg your forgiveness for quoting myself:
This is a complicated issue, and one that is still not fully understood by linguists, or so I believe. In short: there is a tendency in many languages for words to shift in meaning between probability and desirability. This tendency is apparently strongest in certain verbs that are used without specifying who the judge is, which includes passive verb forms.
— Why do we say “was supposed to” for “should have”?
The gerundive always has this double, shifting connotation. In some contexts one kind of modality prevails, in other contexts the other. And with dominant gerundives, such as in gerundive constructions, neither seems to prevail:
Floribus capiendis Eurydice mordetur serpente.