The agreement between the Ceredigion library and Cardiff University means the public will be able to access resources from a new base in the Arts and Social Studies Library.
Previously they were only available on site in Aberystwyth.
There are also plans for joint exhibitions, lectures and events.

The Federal Anti-Monopoly Service (Fas) said that Apple's local subsidiary told 16 retailers to maintain the recommended prices of phones in the iPhone 5 and iPhone 6 families.
Non-compliance with the pricing guidelines may have led to the termination of contracts, it found.
Apple has not yet responded to a request for comment.
At the time of the investigation, Apple denied that it controlled its products' pricing, telling Reuters that resellers "set their own prices for the Apple products they sell in Russia and around the world".
The regulator said Apple had now ended its price-fixing practices but has not said whether the company faces a fine.
The FAS claimed that Apple Rus monitored the retail prices for the iPhone 5c, 5s, 6, 6 Plus, 6s and 6s Plus.
"In the case of the establishment of 'inappropriate' prices, the Russian subsidiary of Apple sent emails to resellers asking them to change," the watchdog said.
The deputy head of the FAS, Andrey Tsarikovsky, added that "Apple actively co-operated" with the investigation and that the company had "adopted the necessary measures to eliminate violations of the law".
That included training employees in the "anti-monopoly legislation norms" in Russia.
In May, the FAS found that Google used its dominant position to force its own apps and services on users and fined it Â£5m ($6m).
And, in November, the regulator opened an investigation into whether Microsoft abused its position in the security market with Windows 10, following a complaint from Moscow-based anti-virus firm Kaspersky.
People will be able to access the National Library of Wales' digital collections from Cardiff under a new agreement.
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Russia's competition watchdog has found that Apple fixed the prices of certain iPhone models sold in the country.