Personally, I would start on the oldest one you can find that isn't a novelisation. Peril at Delphi would be the oldest. My reason for this is that the books go in order of his age.
If you're looking for a good book, my personal favorite would be either Rob MacGregor's last book, Interior World, or Max McCoy's last book, Secret of the Sphinx. The Interior World is, in my opinion, a better book (slightly), but Secret of the Sphinx captures the "essence" (if you will) of Indy.