Calder, George, Auraicept na n-Éces: The Scholar's Primer, Four Courts Press, 1995
This book is bilingual in English and Old Irish, containing some rough going, but it has the ogam tracts from the Book of Ballymote and the Yellow Book of Lecan. These are two of several ogam tracts from original sources, and are essential for understanding the medieval ogam tradition in Ireland. Some of the most essential material from this book can be found in:

Matthews, Caitlin & John, The Encyclopaedia of Celtic Wisdom: A Celtic Shaman's Sourcebook, Element, 1994
This book should, on the whole, be treated with caution, but chapter 2, "The Memory of Trees" simply reprints the material from the ogam tracts and an early article by Charles Graves on ogam, with no comment. Graves himself should be taken with a few grains of salt. He was one of the original researchers of ogam, and much scholarship has passed under the bridge since his works were first published.

McManus, Damian, A Guide to Ogam, An Sagart, 1997
Now out of print, this book is probably the best thing ever written on ogam. It covers much that will be of only minimal interest to Pagans (lengthy descriptions of every individual ogam stone known, long linguistic analysis of the names inscribed on each stone, etc.), but essential material is found in chapter 3, sections 14 and 15, regarding the ogam letter names, and in appendix one, which gives three of the main lists of word ogams. There is a copy of this book in the University of Washington Suzzallo library, or was at my last check there.

MacAlister, R. A. Stewart, The Secret Languages of Ireland, Cambridge University Press, 1937
This is a lengthy study of obscure Irish languages that includes a good bit of information about ogam as medieval code work, and a discussion of Bog-Latin or "ogamized Irish." Worth wading through the ogam chapter if you're fascinated by cryptography and codes.

O'Boyle, Sean, Ogam: The Poet's Secret, Gilbert Dalton, 1980
This explores ogam as harp and musical notation. It's a fascinating theory and well worth taking a look at, if you can find a copy. It was printed by a very small press, and is only sometimes available used. You'll need some musical theory to really appreciate this one, but it does purport to explain some of the more obscure ogams listed in the ogam tracts.

Meroney, Howard, Early Irish Letter Names, Speculum, Vol XXIV, Number 1, January 1949, pp 19-43
This is the work that helped inspire McManus in his work on the meanings of the ogam letters. Some information in here contradicts McManus, and in some cases I prefer Meroney's readings of the meanings. He provides more information on the forfeda or final five letters than McManus does. Very useful. I photocopied the article from the copy of the journal in the Suzzallo library.

Sims-Williams, Patrick, The Additional Letters of the Ogam Alphabet, Cambridge Medieval Studies, Number 23, Summer 1992, pp29-75(?)
This article is excellent for those who are interested in the history and development of the forfeda or final five ogam letters. It has some discussion of the letter name meanings, but most of the article is linguistic and historical in nature. My copy may be missing a few pages from the end of this article, but it was also found in the Suzzallo library.