The noun 书籍 (shūjí) means "books" and is a more formal or literary synonym for 书. It is commonly used in written Chinese to refer to books as a category, such as in libraries, collections, or discussions about reading and publishing.
图书馆里收藏了大量珍贵的书籍。
túshūguǎn lǐ shōucáng le dàliàng zhēnguì de shūjí.
The library has a large collection of precious books.
她从小就喜欢阅读各种书籍。
tā cóngxiǎo jiù xǐhuān yuèdú gè zhǒng shūjí.
She has loved reading all kinds of books since she was young.
这家书店的书籍种类非常齐全。
zhè jiā shūdiàn de shūjí zhǒnglèi fēicháng qíquán.
This bookstore has a very complete selection of books.