Marsius, I saw The Xth Legion at the amazon.com site and was curious, but never got around to buying it.
I found his thesis that Caesar raised the Xth when propraetor of Hispania Ulterior a bit far-fetched. The Xth was in Transalpina when Caesar first got there, so either it was raised by him in accordance with the Senate's edict that he be given governorship of Transalpina with one legion, or it was raised by a previous governor (Celer or Pomptinus). There was a two year gap between Caesar's propraetorship and his proconsulship and the Xth would hardly have been stored somewhere waiting for him.

I have Lawrence Keppie's The Making of the Roman Army about the legions of the late republic/augustan era, and found it very informative.

Perhaps the reviews at amazon.com will be helpful for you in deciding whether to buy.