The whole manuscript has been photographed by the University of OXford and can be viewed online, FREE, at the Bodleian Library website here.
For a recent in-depth study of this codex, see Mark Cruse, Illuminating the Roman d’Alexandre – Oxford Bodleian Library, MS Bodley 264: The Manuscript as Monument (Cambridge: D.S. Brewer, 2011).
For a technical description of the manuscript, see Gisela Guddat-Figge Catalogue of Manuscripts Containing Middle English Romances (Munich: Wilhelm Fink Verlag, 1976), pp.
To learn more about the Middle English poem, see this database entry from the University of York.
You can read the whole of the English poem from this manuscript in a rather out-dated edition online here. The most recent (but not very recent) edition is F.P. Magoun, ed., The Gests of King Alexander of Macedon. Two Middle English Alliterative Fragments, Alexander A and Alexander B. Edited with the Latin Sources Parallel (Cambridge, Mass.: Harvard University Press, 1928).
Two works on Alexander the Great in medieval Britain: Gerrit H.V. Bunt. Alexander the Great in the Literature of Medieval Britain (Groningen: Egbert Forsten, 1994), and Luann M. Kitchel, A Critical Study of the Middle English Alexander Romances. (Dissertation: Michigan State University, 1973).
You can read the Roman d’Alexandre in French (Old French with a translation into modern French) in Le roman d’Alexandre ou Le roman de toute chevalerie, eds. Catherine Gaullier-Bougassas, Laurence Harf-Lancner, Brian Foster and Ian Short (Paris: Honoré Champion, 2003).