Bushey library will be closed for a three-month refurbishment from Saturday April 23.
A temporary mobile library will open in its place while work is carried out.
The work will improve the building's layout, add four additional computers and a public wi-fi facility, with the library due to reopen on Monday July 25.
Hertfordshire County Council said the long closure is to allow for structural alterations to be made to the inside of the library.
It is hoped the alterations will create more space for a new children's area and other library users.
The layout of the library's books will also be changed to allow more space for a new computer area and extra space for events.
Andrew Bignell, head of libraries at Hertfordshire County Council, said: "While we understand that the closure of the library is frustrating for people in Bushey, it is necessary to make these improvements.
"We have tried to include something for everyone in the refurbishment."
The mobile library will be open from Tuesday, May 3, until Saturday, July 16, in the library car park.
Opening times will be Monday to Friday, from 9am to 5pm, and Saturdays, 9am to 4pm.
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