"Liber" can also mean chapters within a book.
Someone else beat me to this elsewhere, but back in Rome's heyday, they didn't have what we think of as books, bound on one side that fit neatly on a shelf. Rather, scrolls were the "book" of the day, and instead of nice, long shelves like we use nowadays, they would store them in cubbies which were highly inefficient.
Books as we know them started out as codexes (codices?) Which would generally be a bunch of papers bound on one edge and protected with a wood or leather cover. These were popularized by early Christians as they made for easy use in reference and travel. And nice, neat libraries, with satisfying efficiency.
"Volumen" is where we get "volume" from, generally denoting an individual book in a series, though we also use it pretty interchangably with "book."
It would be nominative, as a subject, but nominative and accusative have the same form here, in the exercise sentence!
Est volumen primum longum: There is a first long book, or "it is".
Volumen primum est longum (or longum est): The first book is long.
(I'm not sure at all)
Volumen is a neuter noun, therefore the adjectives that modify it have the nominative singular neuter ending, -um (since these are adjs. of the 1st/2nd declensions). (Longus and primus could describe a masc. sing. noun like liber, book; longa and prima could describe a femin. sing. noun like epistula, letter.)