What is Book?

Legal Definition
A book is a set of written, printed, illustrated, or blank sheets, made of paper, parchment, or other materials, fastened together to hinge at one side, with text and/or images printed in ink. A single sheet within a book is a leaf, and each side of a leaf is a page. A set of text-filled or illustrated pages produced in electronic format is known as an electronic book, or e-book.

Books may also refer to works of literature, or a main division of such a work. In library and information science, a book is called a monograph, to distinguish it from serial periodicals such as magazines, journals, or newspapers. The body of all written works including books is literature. In novels and sometimes other types of books (for example, biographies), a book may be divided into several large sections, also called books (Book 1, Book 2, Book 3, and so on). An avid reader of books is a bibliophile or colloquially, "bookworm".

A shop where books are bought and sold is a bookshop or bookstore. Books can also be borrowed from libraries. Google has estimated that as of 2010, approximately 130,000,000 distinct titles had been published. In some wealthier nations, printed books are giving way to the usage of electronic or e-books, though sales of e-books declined in the first half of 2015.
-- Wikipedia
Legal Definition
A general name given to every literary composition which is printed; but appropriately to a printed composition bound in a volume.

2. The copyright, (q. v.) or exclusive right to print and publish a book, may be secured to the author and his assigns for the term of twenty-eight years; and, if the author be living, and a citizen of the United States, or resident therein, the same right shall be continued to him for the further term of fourteen years, by complying with the conditions of the act of Congress; one of which is, that he shall, within three months after publication, deliver, or cause to be delivered, a copy of the same to the clerk of the said district. Act of February 3, 1831. 4 Sharsw. cont. of Story's L. U. S. 2223.
-- Bouviers Law Dictionary
Legal Definition
1. A general designation applied to any literary composition which is printed, but appropriately to a printed composition bound in a volume. Scoville v. Toland, 21 Fed. Cas. 864.

2. A bound volume consisting of sheets of paper, not printed, but containing manuscript entries; such as a merchant's account-books, dockets of courts, etc.

3. A name often given to the largest subdivisions of a treatise or other Uterary composition.

4. In practico, the name of "book" is given to several of the more importent papers prepared in the progress of a cause, though entirely written, and not at all in the book form; such as demurrer-books, error-books, paper-books, etc. In copyright law, the meaning of the term is more extensive than in popular usage, for it may include a pamphlet, a magazine, a collection of blank forms, or a single sheet of music or of ordinary printing. U. S. v. Bennett, 24 Fed. Cas. 1,093; Stowe v. Thomas, 23 Fed. Cas. 207; White v. Geroch, 2 Bam. & Aid. 301; Brlghtley v. Littleton (C. Ct) 37 Fed. 104; Hoimes v. Hurst, 174 U. S. 82, 19 Sup. Ct 606, 43 L. Ed. 904; Clementi v. Goulding, 11 East, 244; Clayton v. Stone, 5 Fed. Cas. 999.
-- Black's Law Dictionary