Yes, BOOK is a Scrabble word!

Letter values in Scrabble US: B 3, O 1, O 1, K 5.

Is it a valid word?

BOOK is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a written work or composition that has been published (printed on pages bound together)
physical objects consisting of a number of pages bound together
a compilation of the known facts regarding something or someone
a written version of a play or other dramatic composition; used in preparing for a performance
a record in which commercial accounts are recorded
a collection of playing cards satisfying the rules of a card game
a collection of rules or prescribed standards on the basis of which decisions are made
the sacred writings of Islam revealed by God to the prophet Muhammad during his life at Mecca and Medina
the sacred writings of the Christian religions
a major division of a long written composition
a number of sheets (ticket or stamps etc.) bound together on one edge

Also: record book, account book, book of account, rule book, al-qur'an, christian bible, good book, holy scripture, holy writ, word of god.

Not playable in Scrabble US: PLAYSCRIPT, KORAN, QURAN.