power of an oath lies in the mindset and not on the book we touch...

To take oath touching a book is not an Indian ritual or way at all..

Of course there were people who invoked the powers of nature or even their own penance or Tapas when trying to reinforce a promise..

The marriage--kanyakaadaanam is done invoking the name of the supreme being ( Narayana Parameshwara Harihara) who bears the weight of the universe and also invoking the witnessing of all the Gods, with the following mantram

विश्वम्भरः सर्वभूताः साक्षिण्यः सर्वदेवताः।

इमां कन्यां प्रदास्यामि पितॄणांतारणाय च।।2।।

(The Lord who carries the Universe, all the elements, all the Gods, are witnesses.. I am giving this girl in marriage to you so that the elders of my race attain eternal pleasure)

While being initiated to Gayatri and study of Vedas, the promise that one shall perform the duties as a brahmachari for ever is made by touching the chest or the heart region of the guru with the right hand. and making a solemn promise..

The promise that one will look after the girl received in marriage with all happiness and safety is also given by the groom placing the finger on the chest of the person who gave he Kanya to him.

When someone takes charge of a new office, according to the Indian traditions the elders and the sages present just instruct the employee, be it king or the minister, about his duties and is offered aasheervachanams by pouring grains and valuables and holy waters on the prostrate figure of the would be employee. who offers his pranams to all elders.

So making a fuss about taking oath on Gita or any printed book is basically of little consequence anyway..

In fact the procedure for oath is that the person being sworn in invokes the name of God or Solemnly affirm that he will be faithful to his motherland and would discharge his duties without fear or favour. and this oath is taken in the presence of an authority who is empowered to witness such oath.. The President, The Speaker, The Chief Justice, Authorities in Law, Quasi Judicial authorities and so on. ( There is a law for oaths in India)

The showing of a book, as I could understand from old colonial literature was just a sort of gimmick of the westerners..

I think you are aware that the much announced news item about some US judge taking oath on Gita was just a media hoax..