Hughes' widow Carol said: "Ted was a man of these islands - their landscapes, rivers and wild places - and it is fitting that papers covering such an important part of his creative life should be deposited with such a prestigious institution here in Britain."
Birthday Letters, published only months before Hughes' death in 1998, was Hughes chronological account of his relationship with Plath.
It sold more than 500,000 copies worldwide and received both the TS Eliot Prize for Poetry and the Whitbread Book of the Year award.
Hughes penned some of his drafts in school notebooks
Elsewhere, a "mind map" sees the poet organising his thoughts and themes on his wife's life in a diagram-form.
He draws out themes in a spidery carousel - jealousy, critics, fairy godmother - jotting down ideas that germinated into verse.
"He started with little one-sentence aide memoires, then wrote prose and worked that into poetry," says Andrews.
"But there was an in between phase where he worked out the ideas on a variety of astrological charts.
"He was trying to tie them into some kind of supernatural, super-mystical tradition."
Diaries, letters and correspondences with translators feature in the archive
By the time he had readied Birthday Letters for publication, Hughes' jagged handwriting had become more expansive and relaxed - suggesting a man more at peace with his demons.
In personal letters, he says it had been necessary to write about his tragic relationship with Plath to "relieve the log jam" of his creativity.
Evidence of this comes in the numerous uncompleted poems he was working on at the time of his death from cancer in 1998 - also included in the archive.
The collection was purchased from Hughes' estate for £500,000 - including a £200,000 donation from The Friends Of The British Library - and consists of more than 220 files and boxes of manuscripts.
"We were really bowled over to find out how much there was," says Andrews.
Aside from the Birthday Letters manuscripts, scholars will have access to journals, personal letters, dream diaries and correspondence with critics and translators.
The range and extent of the material on offer rivals - and may even eclipse - the collection housed at Emory University in Atlanta, Georgia, which includes Hughes' personal library and letters to his lover, Assia Wevill.
Andrews says the British Library hopes to work with the American institution to further the study of Hughes' life and work.
The library's acquisition includes funding to catalogue and preserve the Hughes collection, which is expected to be fully available for research by the end of 2009.
In the meantime, it hopes to keep a blog chronicling the cataloguing process, as it has done for the Harold Pinter archive that was acquired in December 2007.
A notebook containing early drafts of Birthday Letters, revealing that Hughes had originally planned for the volume to be entitled The Sorrows of the Deer, will also be displayed in library from 15 October 2008.
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