Dux,
I too have been researching that period and the pre-Roman cultures of Iberia as well. The Celtiberians were a mix of Gallic Celt and some mysterious pre-IndoEuropean group that may be best likened to the language of the Basque, but when attemptes were made to apply Basque syntax to ancient Celtiberian inscriptions, the translations failed. There are inscriptions found on pottery and funerary masonry, and some of the inscriptions are in a Latinised alphbet, while others employ a more semetic system such as Phoenician. Though the Ionic Greeks colonised that penninsula as well, their alphbet did not become widespread. Of course the tribes applied other scripts that met their spoken language sounds, but each fell short to some extent because not all sounds were properly represented in borrowed foreign script. Below are some books I might recommend you strat with and of course the subject and period of time has spawned a wide variety of academic works in archaeology, history, and linguistics. Let me know if you have any other questions about that era in Iberia!
1. Books that primarily deal with Iberian Prehistory/protohistory:
A. Castro, Maria C.F. Iberia in Prehistory. ISBN #0-631-16794-3
B. Ruiz, Arturo and Manuel Molinos. The Archaeology of the Iberians. ISBN#0521564026
C. Harrison, Richard J. Spain at the Dawn of History. ISBN#?, LCC#87-51688--Has a good chapter (although dated) on the langauges of the Celtiberians.
2. Books with chapters dedicated to Iberia:
A. Maier, Bernard. The Celts. ISBN#0-268-02361-1
B. Rankin, H.D. Celts and the Classical World. (there might be a newer edition than this 1987) ISBN#0-918400-06-6
C. Green, Miranda J. (Now Miranda Aldhouse Green) {editor}. The Celtic World. ISBN#0415-05764-7 (recommend the hardback version as it is a monster volume and a paperback probably wouldn't last very long) *especially note Part I (where you will see Rankin author of above mentioned book listed as a contributor), Part IX (pgs. 533-550) and maybe Part XII (pgs. 804-814)
An additional book I recommend to anyone interested in the Celts is below:
Cunliffe, Barry. Facing the Ocean. There's a pretty good qualtiy paperback of this book, but here's the infor for the first edition: ISBN#0-19-924019-1