I know, this is supposed to be printed sources, but do check Every Ogham Thing on the Web and TITUS-Ogamica, a scholarly site about the inscriptions themselves, with bibliography and images. Although I’m less fond of Graves’ The White Goddess, this site has some other resources.
Calder, George ed.“Auraicept na n-Eces” from The Book of
Ballymote. Edinburgh: John Grant.
Carney, James. “The Invention of the Ogam Cipher.” Ériu 26 (1975):53-65.
Jackson, H. Kenneth. (1950). “Notes on the Ogham Inscriptions of Southern Britain.” Chadwick Memorial Studies: Early Cultures of Northwest Europe. Eds. Sir Cyril Fox and Bruce Dickins. Cambridge: Cambridge University Press, 1950.
McManus, Damian. A Guide to Ogham. (Maynooth Monograph 4) Maynooth: An Sagart, 1991. This is the book to start with; it offers a good introduction and theoretical discussion of ogham issues and it has an extensive bibliography.
McManus, Damian. “Irish Letter-Names and their Kennings.” Ériu XXXIX (1988): 127-68. This is an edition with full text and commentary, of the Old Irish Briatharogaim, the kennings associated with the letter-names. McManus demonstrates quite effectively that most of the tree-alphabets are a late addition by the medieval glossators. You might also want to take a look at his “Ogam: Archaizing, Orthography and the authenticity of the manuscript key to the Alphabet.” Ériu (1986):1-31.
MacNeill, John. “The Irish Ogham Inscriptions.” Proceedings of the Royal Irish Academy, Vol. 27, Section C, 329 ff. (1909). The oldest documents of all in Irish, the stone inscriptions in Ogam, are discussed as the earliest evidence of Irish grammar.