book这个词应该是相当的可数（countable）。
它在作“图书”（a set of printed pages that are held together in a cover so that you can read them）、“笔记本”（a set of sheets of paper held together in a cover so that you can write on them）、“册/集”（a set of things such as stamps, matches, or tickets, held together inside a paper cover）的时候，是绝对的可数。
有时候，必须要用复数，也是可数的表现，比如：当表示“account”的时候（written records of the financial accounts of a business），多用复数，e.g.An accountant will examine the company's books.
还有一些固定搭配，也可以体现book是可数的：
by the book：exactly according to rules or instructions
a closed book：a subject that you do not understand or know anything about
但也有一些情况，他并没有配冠词，而是单独使用，非常少见，例如：
in my book：【spoken】said when giving your opinion:
e.g. In my book, nothing is more important than football.
bring somebody to book: to punish someone for breaking laws or rules, especially when you have been trying to punish them for a long time:
e.g. War criminals must be brought to book.
所以，总的说来，book是比较确定的可数名词。
参考资料：longman dictionary