Dear Prof. Paturi,
These 'ancient pastoral nomads' were pre-literate. They didn't write any horse-training books. The horse-training book was written by someone named Kikkuli, and he wrote it in Hittite, not Vedic. In his Hittite horse-training manual there are some clearly pre-Vedic words that refer to horse-training and certain Vedic gods. Kikkuli was transcribing into Hittite the words of these pre-Vedic nomads who taught the Hittites about horse-training.
The article that Dominik refers to is a a rather teasing critique of Modi, et al. It is not scholarly, but it gets the facts right for the most part. Its main point is that the OIT theory is not supported by any good evidence. It cites reliable sources like Anthony and the encyclopedia of IE by Mallory and Adams. If you want to consider what Vedicists think about this, see Mayrhofer and Thieme, just as a start.