Our information books are classified according to the Dewey Decimal System (see below) and are shelved in numerical order. These classification numbers are also included on the catalogue record for each title

Each library has a printed copy of the ‘Subject Index’, which lets you look up any subject and find the relevant Dewey number

Dewey decimal system
A system for organizing the contents of a library based on the division of all knowledge into 10 groups, with each group assigned 100 numbers. The 10 main groups are:

000–099, general works
100–199, philosophy and psychology
200–299, religion
300–399, social sciences
400–499, language
500–599, natural sciences and mathematics
600–699, technology
700–799, the arts
800–899, literature and rhetoric
900–999, history, biography, and geography

These 10 main groups are in turn subdivided again and again to provide more specific subject groups

Within each main group the principal sub-series are divided by 10; e.g., the history of Europe is placed in the 940s. Further subdivisions eventually extend into decimal numbers; e.g., the history of England is placed under 942, the history of the Stuart period at 942.06, and the history of the English Commonwealth at 942.063

This explanation is reproduced, with amendments, from"Dewey Decimal Classification" Encyclopædia Britannica  from Encyclopædia Britannica Online.