If you are not already familiar with Catullus, read as much as you can in Latin beforehand. This will make things less exhausting during the workshop itself.

Suggested books (buy as many as you can afford; they are listed more or less in order of importance):

Mynors, R.A.B. 1958. Catulli Carmina. Oxford: Oxford University Press.

This is the standard text, and the one we will use to translate from. Pricey new, but it is available used for around $15 from sites like abebooks.com.

Garrison, Daniel H. 2004.The Student’s Catullus. 3rd ed. Norman, OK: University of Oklahoma Press.

A reasonably good student edition with some grammatical help.

Green, Peter. 2005. The Poems of Catullus: A Bilingual Edition. Berkeley: University of California Press.

A nicely produced edition with an excellent translation and intelligent interpretive notes in the back.

Quinn, Kenneth. 1970. Catullus: The Poems. London: Macmillan.

The classic student edition with some grammatical help by one of the finest Catullan scholars of the 20th c.

Fordyce, C.J. 1961. Catullus: A Commentary. Oxford: Oxford University Press.

More detailed, and less grammatical help than Quinn, but very good on the poems he treats. The obscene ones are omitted.

Thomson, D.F.S. 1997. Catullus. Toronto: University of Toronto Press.

New, bigger, but arguably less useful than Quinn or Fordyce; very full on textual matters. Extensive bibliography on each poem.

Make sure you have a good dictionary, at least a full as:
Simpson, D.P. 1968. Cassell's Latin Dictionary. New York: Wiley Publishing, Inc.

If you are already familiar with Catullus and want a good book to read in preparation, check out:
Kolson Hurley, Amanda . 2004. Catullus. London: Duckworth.

Areadable overall introduction by the assistant book editor of the Washington Times.

If you want something more ambitious, check out:
Wray, David. 2001. Catullus and the Poetics of Roman Manhood. Cambridge, Cambridge University Press.

A sophisticated but reasonably witty discussion of many of the most familiar poems by a sensitive critic.

More good suggestions:

volatrix would also like to suggest the following:

Lee, Guy (trans), Catullus The Complete Poems, Oxford World's Classics. (this is a bilingual edition)
and
Skinner, Marilyn B., A Companion to Catullus, Blackwell Publishing. This is expensive and my internet search indicated that inexpensive used copies did not abound!