This is the editio princeps of the treatise 'On the Old and New Testament' by Ælfric of Eynsham (c. 955-c. 1010). Despite what is said in the book title, he was not Archbishop of Canterbury. The work was originally a letter addressed to a Sigeweard, and is one of Ælfric's last works; thus, despite what the book title says, it was not written about the time of King Edgar, who died in 975. Ælfric teaches Christian doctrine and reviews the Old and New Testament, book by book.
Nothing is known about the recipient of the letter, Sigeweard, but judging from the information given in this letter, he seems to have been a nobleman having a house in East Halon, located east of Eynsham, where Ælfric was an abbot. He often asked Ælfric for some text, while at some point he invited him to his own house, served some alcoholic drink and ended up trying to persuade him into drinking too much (for which Ælfric blames him near the end of the letter).
The treatise is preserved, fully or partially, in four manuscripts. Oxford, Bodleian Library, Laud Misc. 509 is the only manuscript recording a complete text; the other three manuscripts, Oxford, Bodleian Library, Bodley 304, London, British Library, Cotton Vitellius C.V., and British Library, Harley 3271, preserve fragments or short excerpts of it.
The editor, L'Isle, provides the original Old English text based on Laud Misc. 509 and its English translation on facing pages. L'Isle's main purpose to publish this book is to preserve 'an ancient monument of the Church of England' (p. b1r) and provide it to the readers. In the preface, he writes that he is 'not able to set out any new matter of that kinde [i.e. of Christian Religion] worth reading' (p. b4r) and also that it is unnecessary for him to write anything new 'in this age of full of good writers' (p. b4r). So he decided to apply his diligence in preserving the learning in the 'orchard of the old English Church' so that he 'may saue at least a good old tree or two, that were like to be lost' (p. b4r). It seems that this was part of his larger preservation project, as he also writes that this book is 'for a triall' before editing a part of the Bible translated into Old English.
In the preface, L'Isle also writes how he learned to read Old English. He started by acquainting himself 'with the Dutch, both high and low' and then he read 'a while for recreation all the old English' (p. c4v) he could find, poetry or prose. He also read 'Virgil Scotished by the Reuerend Gawin Dowglas' (p. c4v), with the help of its original Latin version. He says that he 'got more knowledge of that I sought than by any of the other' from this book in Scots, as it is 'neerer the Saxon, because farther from the Norman' (p. c4v). He also mentions Henry Spelman and Robert Cotton as his helpers.
Apart from the letter to Sigeweard, the volume also contains a homily for Easter Day from the second series of Ælfric's Catholic Homilies, his letter to Wulfsige (in Old English), his letter to Wulfstan (in Latin), the Lord's Prayer, the Creed, and the Ten Commandments (in Old English and English).
A newer edition of the work is included in S. J. Crawford, ed., The Old English Version of the Heptateuch, Ælfric's Treatise on the Old and New Testament and his Preface to Genesis, EETS o.s. 160 (1922; repr. with the text of two additional manuscripts, London: OUP, 1969), in which L'Isle's translation is reprinted, whereas the latest edition with a new translation is L. J. Swain, ed. Ælfric of Eynsham's Letter to Sigeweard: An Edition, Commentary and Translation (Troy, AL: Witan Publishing, 2020), which is based on the author's PhD thesis of the same title submitted to the University of Illinois at Chicago in 2009.