One head teacher decided "all reading can be done on iPads," a delegate told the Association of Teachers and Lecturers' annual conference.
The union voted to lobby for libraries to be included in Ofsted inspections.
A Department for Education spokeswoman said that school libraries "played a vital role".
Cathy Tattersfield from Derbyshire quoted international evidence suggesting a positive correlation between good school libraries and student attainment.
Ms Tattersfield said she had been "shocked" that two secondary academies had "recently closed or attempted to close their libraries and several of them have had their librarian hours or posts cut, mostly in the ex-mining areas of Derbyshire".
A survey of 485 ATL members last month suggested a patchy picture on library provision said Ms Tattersfield.
"It seems to be feast or famine.
"Some secondary provision is fine, cherished and secured," - but others faced reduced opening hours, conversion to e-learning centres, cuts in staff hours, or librarians having been removed or replaced by support staff or teachers.
"We identified a third of secondary schools have had cuts of 40% or more since 2010, with 20% redundancies in library staff at their school."
Another delegate said his school library, closed last year, had provided a host of benefits to pupils, from paired reading for special needs pupils to introducing able readers to new authors and running a range of enrichment activities.
He said the library's books and magazines had been distributed among subject departments.
"Lesley Mumbray-Williams whose school dispensed with her services as librarian said a third of the stock had ended up in skips within three weeks of her leaving."
Another delegate said the librarian post in their school had not been filled and the library doors were often locked.
Of the education staff who responded to the survey:
A Department for Education spokeswoman said: "We want all children to read widely and well and believe school libraries can play a vital role in fostering that love of reading.
"We trust schools to decide on whether to provide and maintain a library service for their pupils."

The UK will maintain an EU deal, which provides 48 countries with duty-free access to Britain for imports.
It means British firms do not pay import tariffs on goods bought from countries such as Bangladesh and Haiti.
International Trade Secretary Liam Fox said Brexit gave the country an opportunity "to step up our commitments to the rest of the world".
He added: "Free and fair trade has been the greatest liberator of the world's poor, and today's announcement shows our commitment to helping developing countries grow their economies and reduce poverty through trade."
The deal excludes arms and ammunitions.
The list of countries - which also includes Ethiopia, Sierra Leone, and Uganda - is based on the UN's Least Developed Countries index.
Currently Â£19.2bn of goods are imported from the 48 countries, including 79% of the tea consumed in Britain.
Some 45% of the UK's textile and clothing imports, and 22% of its coffee, also come from the developing nations.
International Development Secretary Priti Patel said the renewed commitment would "help the world's poorest people stand on their own two feet".
She added: "Building a more prosperous world and supporting our own long-term economic security is firmly in all our interestsâ€Ž."
The government also intends to explore options to expand relationships with richer countries like Jamaica, Pakistan and Ghana, which currently enjoy a mixture of reduced tariffs or zero tariffs.
Dr Fox added it would result "in lower prices and greater choice for consumers".

The 75-year-old was recovered unconscious from the water at Tywyn, by a crew from Aberdovey RNLI Lifeboat at 16:14 BST on Saturday.
He was flown to hospital by the coastguard rescue helicopter but later died.
The girls were taken to hospital as a precaution.
In a separate incident, three people including a 16-year-old boy and a 14-year-old girl, were rescued from the sea in Harlech at about 15:30 BST on Saturday.
Harlech Coastguard team, Criccieth RNLI Lifeboat, North Wales Police and paramedics were involved in the rescue.
North Wales Police warned people to take extra care in the sea and to avoid taking risks.
Too many school libraries in England face cuts or closure with schools increasingly viewing books as obsolete, a teachers' union has heard.
[NEXT_CONCEPT]
Free trade deals with developing countries will continue post-Brexit, the government has said.
[NEXT_CONCEPT]
A man has died after attempting to rescue two teenage girls who got into difficulty in the sea in Gwynedd.