Library users will be able to choose their books on a Sunday for the first time in the Royal Borough starting next April.
The container library at Sunningdale will also be included in the scheme, which will be rolled out to other libraries if successful.
Cabinet has approved an extra £25,000 to be used either to pay for more staff or to pay existing staff for working on Sunday if they choose to do so.
Longer opening hours on other days are also proposed.
Already members of the union UNISON are worried.
Vice-chairman of the Windsor and Maidenhead branch of UNISON, Alan Barwise, said: "We are concerned about the detrimental impact on staff. They could be taken away from their families on Sunday.
"The council is already stopping the bonus that used to be paid for working Saturdays in our libraries so we have no idea if it is intended to compensate them if they have to work Sunday. We have not been consulted about any of it."
He pointed out that it would have been inconceivable even five years ago for a local authority to consider extending a service like libraries, without employing professionals to take on the extra duties.
He said: "Of course library work can be done by volunteers. But running a library is more complicated than that.
"Sometimes the Big Society can be dangerous.
"The constant talk about multi-skilling can show a lack of understanding about how professionals work."
Royal Borough cabinet members gave the go-ahead last week to plans for a new library at Boyn Grove. It will be based in a side extension adjoining the Community Resource Centre and staffed with the help of volunteers and people with a learning disability, through the Ways into Work programme which is based at the centre.
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