This book is based on Beowulf, a work that exists in a sole manuscript called Cotton Vitellius A XV, housed at the British Library.
It dates from around 1000 and was partially destroyed by fire in 1731. That we have it at all attests to the tenaciousness of both the manuscript itself and scholars interested in preserving it. There continues to be much debate about the dating of the original version of Beowulf, since it is generally assumed by scholars that the one text we have is a late version of a poem originally orally composed.
The original poem exists in one manuscript at the British Library. As the BL page tells us, the manuscript contains much more than the Beowulf poem. It also contains “four separate items, bound together for Sir Robert Cotton (d. 1631):(i) f 1: Psalter leaf (now removed to form London, British Library, MS Royal 13 D I*, f 37); (ii) f 3: Medieval endleaf, containing historical memoranda; (iii) ff 4–93: Augustine of Hippo, Soliloquia (ff 4r–59v: imperfect); Gospel of Nicodemus (ff 60r–86v: imperfect); Debate of Saturn and Solomon (ff 86v–93v); homily on St Quintin (f 93v: imperfect); (iv) ff 94–209: Homily on St Christopher (ff 94r–98r: imperfect); Marvels of the East (ff 98v–106v); Letter of Alexander to Aristotle (ff 107r–131v); Beowulf (ff 132r–201v); Judith (ff 202r–209v: imperfect). F 2 is a 17th-century Cottonian endleaf.”
You can actually flip through the manuscript starting with The Homily on St Christopher here.
As Kevin Kiernan points out on his website, “When viewing Cotton Vitellius A. xv as a manuscript book, the last page of Alexander’s Letter faces the first page of Beowulf in the Nowell Codex.”
For a fabulous list of resources concerning Beowulf, check out this page.