A: It is highly desirable to have a qualified, traditional guru. There do not seem to be many of these around today and it is unlikely that you happen to have one conveniently close by! Consequently, the best you can do is to read books that reliably present material in the traditional manner (unfolding scriptural texts and Shankara commentaries) and listen to recorded talks from similarly reliable sources.
Reading transcriptions of talks by Ramana or Nisargadatta do not satisfy these criteria. The talks were given to particular groups of people and answers given to specific individuals with their unique level of prior (mis-)understanding asking about their difficulty-of-the-moment. The talks are usually translated and reworded by someone unlikely to have the level of understanding of the speaker. Vivekananda, brilliant though he was and grateful though we should be for his introduction of Vedanta to the West, had some critical misunderstandings of Advaita and these are very likely to generate confusion in a seeker.
It is extremely unlikely that anyone would attain enlightenment without following either of those routes. You don’t just happen suddenly to think that the universe is mithyā and who-you-really-are is the non-dual reality! Clearly Ramana et al prove that it is possible, but they presumably had an upbringing in which the teaching was not alien, in the way that it is now in the West.
Advaita recognizes all ‘paths’ as valuable preparation but the fact is that only knowledge can give enlightenment. This must be so if you think about it – reality is already non-dual but we initially do not know this. So by all means practice mediation, worship gods etc. Such things help concentrate and still the mind so that it can give attention to the teaching. But don’t expect any other source to give enlightenment.
I, too, found ‘I am That’ to be inspiring – many years ago now. But even that is a source of confusion with its total lack of systematic organization and its Q&A presentation. Traditional teaching presents a topic in a proven manner and you ask questions afterwards to clear any doubts. I rather think that the idea that one cannot achieve enlightenment in a single lifetime is given to ensure that only the really serious commit to prolonged teaching from a qualified guru! If it is the most important thing in your life and you devote commensurate time to studying it, one lifetime should be more than adequate!