Acknowledged authority: the Guru
in Hinduism, the term Guru refers to a personal spiritual teacher or guide who has himself attained spiritual insight. Linguistically speaking, Guru is a Sanskrit adjective which means "heavy" (cfr. latin gravus), and, by extension "important". In India, due to the lack of a central religious authority like the Church in the West, the Guru has always been the institution of authority for every religious tradition in India. There's no religious denomination in India that doesn't founds its authority and tradition on a succession of Gurus.
The Guru possess the authority regarding religious truths because he himself is the living embodiment of these truths. According to the basic theological tenets of Hinduism, the Hindus religious goal is to discover the pristine union of the inner Self (Atman) with the Absolute (Brahman). Given this context, the Guru, who has already discovered such a union, isn't simply the person who teaches about the Guru, but he is the Absolute itself teaching through the Guru.
As virtually every religious institution throughout the world (like monasteries, for example), the Guru and his (or her) hermitage has been functioning also as a place of social, political and even economical influence. Also in nowadays India is a common custom for politicians and businessmen to pay homage (often with generous donation) to famous Gurus in order to obtain his spiritual approval and thus gaining social acknowledgment.

Sacred Texts and other main texts: Guru-gita
The Guru Gita (Song of the Guru) is a Hindus scripture authored by a mythical sage, Vyasa. Is a chant composed of 352-verses which have been basically transmitted orally and present itself as a part of different larger texts. Therefore is fairly difficult to date.
It describes a conversation between the Hindus god Shiva (see sec. 6) and his wife, the Hindus goddess Parvati, in which she asks him to teach her about the Guru. Shiva answers her by describing the Guru principles, the proper ways of worshiping the Guru and the methods and benefits of repeating the Guru Gita.

Main changes in the XX sec.: the figure of the guru today as the trademark of the indian spirituality
In nowadays India the Guru is the pivotal figure around which a huge marketing of religious goods develops: pilgrimages to Guru's hermitage, donation, Yoga or other spiritual formation courses' fees. In the encounter with the West the cultural elites of India have presented the figure of the Guru as the symbol of traditional culture, as an eternal heritage to offer to the "materialistic" West. In this way this spirituality embodied by the Guru has become the "trademark of India", triggering a sort of "spiritual tourism" still active nowadays.
The West has been receptive to this message particularly during the "Counter Culture" years, in the mid 60's- 70's, also as a protest the old and hierarchical institutions of the Christian Churches. Moreover, this kind of imported spirituality well suited the growing individualism of the consumerist society: each person can pick up his favorite Gurus (from a large great offer) following his/her taste.
Famous Indian Gurus who have extended their messages (and their books!) outside India are worldwide known, and often venerated, persons. Notable example include Satya Sai Baba (1926-2011) and Bhagwan Shree Rajnesh alias Osho (1931-1990).

Analysis and of the sources:
Source n° 1
The Guru Gita
In these four stanzas one can easily discern the characteristics of the figure of the Guru. Stanza n° 18 teaches the disciples about the selfless devotion that has to be paid to the Master, because, as told in the stanza n° 32, the Guru himself is the personification of various Supreme Gods, is a manifestation of the Absolute itself, the Brahman. Stanza n° 9 involves a next step in the training under a Master: the process of interiorization of the Master's teaching, resulting in the discovery of the same innermost nature between disciple and Master (the Atman, the Self, the equivalent of the Absolute in every men). From this realization proceeds the process of merging in the Brahman, involving also some kind of depersonalization ( "one may abide anywhere, firm in that realization"). It is in the nature of Man to merge with the Absolute, the same way it is the nature of the larvae to become a bee. The figure of the Master embodies this concept and following his teaching leads to this result.

Source n° 2
Images of Maharishi Mahes
The first image portraits, together with the famous band "The Beatles", Maharishi Mahesh (12 January 1918 – 5 February 2008) a spiritual master and entrepreneur. He developed the Transcendental Meditation technique and was the leader and Guru of a worldwide organization that has been characterized sometimes as a new religious movement and sometimes as non-religious. The Maharishi is credited to have founded thousands of teaching centers and hundreds of colleges, universities and schools.
In 1967, the Maharishi's fame increased and his movement gained greater prominence when he became the "spiritual advisor to the Beatles", though he was already well known among young people in the UK and already had numerous public appearances that brought him to the Beatles' attention. They met in London in August 1967 and the Beatles went to study with the Maharishi in Rishikesh, India for about six weeks.
Their interest in the Maharishi changed Western attitudes about Indian spirituality and encouraged the study of Transcendental Meditation. The cover of Time Magazine (13 October 1975) suggests the noteworthy relevance of eastern spiritual matters in the West in the years of the so called "Counter Culture" of the 1960s and 1970s.
The Maharishi is credited with heading charitable organizations but also for-profit businesses and real estate investments whose total value has been estimated at various times to range from US$2 to US$5 billion.