The recording of their lives and events was the job of the Rishis. They did it on palm leaves in Sanskrit and it was handed over through the system of Gurukul (teacher- student). All the Puranas, Ithihasas and other works have come to us in that way only.
The first ever ‘boasting’ was done by Ashoka in the language that Vediks of olden times did not approve / did not consider as worthy. The development of writing of a language different from Sanskrit was done by Jains and Buddhists only. So any effort to establish olden past of the Vedic kings and Vedic society by means of inscriptions carved on rocks in Brahmi or Tamil Brahmi scripts will not yield results.
Given below is the conclusion of a review of an article on old writing systems. The entire article can be read in the link given below.
....But can we imagine such a state of affairs, given what we know (admittedly not too much) of the state of society and culture in India, especially in the northeast, before this time? If we can put any trust at all in the traditional lore of the Purâ.nas and the testimony of the Pali canon, Magadha was the site of great and prosperous empires, notably that of the Nandas, decades if not centuries before the foundation of the Mauryan dynasty in around 320 BC. Can we believe that these dynasties with their legendary riches, and the remarkable intellectual and cultural life of India in the time of the Buddha and Mahâvîra, existed in a totally illiterate sphere? It is certainly true that intellectual activity in India has always strongly favored oral over written means of expression, and both von Hinüber and Falk have effectively put to rest the already discredited skepticism about the possibility of oral composition and preservation of the Veda, Pâ.nini's grammar, etc. (see e.g. Falk pp.321--7). But the fact that Pâ.nini did not use writing in composing the A.a.tâdhyâyî does not necessarily mean that he was illiterate (cf. Falk p.259); it may only mean that writing was not considered an appropriate vehicle for intellectual endeavors of his kind. Even given the very different cultural role of writing in India as compared to many other ancient civilizations, it is hard to conceive that practical affairs such as the keeping of records and accounts in a fabulously wealthy empire like that of the Nandas could have been kept in order without any form of writing at all, or at least without some alternative system of memory-aids like the Inca quipu . Thus one is tempted to think along the lines of William Bright (cited by Falk, p.290) of some type of writing that was "perhaps used for commercial purposes, but not for religious or legal texts." 
Admittedly, we have not a shred of concrete evidence for this, and it is perhaps better to stick with what we have and assume that business affairs, like cultural ones, were conducted in pre-Mauryan Magadha simply on the basis of the highly-developed memory skills so well attested in ancient and modern India, perhaps with the assistance of a system of numerical notation such as that hypothesized above. This, it would be hard to deny in light of the evidence that Falk, von Hinüber, et al. have laid out before us, is the most likely scenario on the grounds of the unfortunately meager evidence that is left to us. Still, we should not fall into the trap of thinking that the last word has been spoken. Admittedly, it hardly seems likely, after all the years of waiting, searching, and the dashing of false hopes, that some major archaeological discovery will reveal a whole new picture of the origins of writing in the Indian heartland, or reveal a sustainable (rather than purely hypothetical) connection with the Indus script. Nevertheless, it would be unwise to rule out surprises in the future, and we should leave the door open, as does Falk (p.340), to discoveries that could revive theories of an early development of Brâhmî. But we must also agree, if reluctantly, with his final sentence: "Zur Zeit erscheint dieser Fall jedoch kaum zu erwarten" (p.340).