The Mitchell Library in Glasgow, which opened in 1911, has more than one million items of stock and is a central information hub for people from the city and across Scotland.
The Charing Cross building also holds the Glasgow city archives and collections and is one of the world's best resources for researching family history.
Work will start on a £3.4m upgrade to the building on Thursday and will replace the 30-year-old fire detection system, improve its lighting and install new lifts.
It will also include the creation of a family history centre within the library, an improved study space and two upgraded public lifts.
Councillor Archie Graham, Chair of Glasgow Life, said: "The fire alarm upgrade is essential for public safety and to ensure we safeguard the Mitchell Library's outstanding collections and the rich heritage of the city that is housed within the building. I'm confident the public will be pleased with the outcome when the refurbishment is complete."
There will be limited access to some of the library's collections during the work. Levels four and five will close from Thursday until March 2014. Services from those levels can be requested from staff on levels two and three and can also be requested in advance by calling 0141 287 2999.
Glasgow Life, which operates the Mitchell Library and 32 other libraries in the city, said it had scheduled the work in two phases to minimise disruption.