Is this the future tense of the perfective verb?
завтра они прочитают книгу

If so, how would you translate it?
As 'They will read the book tomorrow'.
Or as 'They will have read the book tomorrow.' ?

See a translation

My 2 cents to you is to forsake, or at least critically consider direct analogies between tenses in different languages, as there's simply no bijection between any two distant enough languages, and as every language has its own innate tendency to govern its grammar according to its very own factors and causes.
The English future perfect does not have a direct analog in Russian, and as such, the Russian perfective future can be used to denote it, but only as a single contextual variant amongst several, when the overlying context fits the bill.

Future perfect serves to express the notion that a certain action will have already been finished by the time a second one occurs, or a future point in time is reached. It's entirely oblivious to whether the action is solitary or repetitive, one-time or habitual, or whether its duration is mentioned.
You can not use the perfective future to translate "they will have read the book 5 times by tomorrow", because that is an explicitly repetitive action, meriting the complex future, e.g: "они до завтра будут читать (прочитывать) эту книгу раз 5". Same goes for "they will have been reading the book for 45 minutes when you arrive", because that is an action whose duration is explicitly delineated. e.g: "они будут читать (прочитывать) эту книгу за 45 минут когда Вы придёте".

On the other hand, the perfective future expresses a completion of a single action, whose nuances are ascribed by the verbal prefix. It is otherwise completely oblivious to any other aspect, feature, or circumstance in which it was done, and lacks the distinct illustrative properties of the English future perfect.
Sans any further context, "завтра они прочитают книгу" only means that "they will read the/a book tomorrow", e.g: "завтра они прочитают книгу, попишут диктант, сдадут экзамен, и потом рано разойдутся по домам". There's even no direct indication of whether the book will be read throughout.

This all goes to show that grammar is arbitrary. One can decide that if something is from New Zealand and walks on all fours, then it's a sheep, and one would, indeed, be mostly right, but would inevitably overlook the occasional house pet, ungulate livestock, or dominated sex slave caught in the act.