"The first book is long."
Translation:Volumen primum longum est.
There's nothing making the word "book" accusative: you'd need a verb (like I have, He reads, She looks for, etc.) that controls a direct object; or you'd need one of the prepositions (like ad, per, prope) that govern an accusative object (He puts a pen near the book).
The adjectives "first" and "long" have 30 forms each (3 genders, m/f/n; 2 numbers, sing. vs. plur.; 5 cases, nomin/gen/dat/acc/abl).
They are not in the accusative! It's true that the ending -um is used in the 2nd decl. for "masc sing accus" (as in, "I hold the first long book": Prīmum librum longum teneō); but the crucial point here is that the ending -um is also used for "neuter singular nominative," as here: Volūmen is a neuter noun (of the 3rd decl.), so it is modified by -um adjectives.
The adjectives are listed as: prīmus, prīma, prīmum; and longus, longa, longum. Those are the 3 nominatives singular, masc/fem/neuter.
(Does that help?)
That would be a so-called 'indirect statement':
Librum longum esse dūcunt .
(The verb of THINKING, SAYING, KNOWING, FEELING, etc.--here, dūcunt in the sense of "consider"--as in "We HOLD these truths to be self-evident"--governs an accus/infin clause, replacing the type of subordinate clause (starting with conjunction THAT) with nominative and indicative, that we use in English.)
Liber est longus is the version in "direct speech" (an actual quote from somebody).