"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": Primum librum longum teneo); but the crucial point here is that the ending -um is also used for "neuter singular nominative," as here: Volumen is a neuter noun (of the 3rd decl.), so it is modified by -um adjectives.
The adjectives are listed as: primus, prima, primum; and longus, longa, longum. Those are the 3 nominatives singular, masc/fem/neuter.
(Does that help?)