There are innumerable books that detail how to do Vedanta Vichăra (Self-enquiry), and Dhyana. Many a book also gives the Mantra for the Japa. While these are so easily available why do we need the guidance of a Guru to perform these?
Sri Sri Muralidhara Swamiji
A well posed question, indeed! You are very right. Today such informations are available in varied details in books. Anyone who is interested in achieving his goal should be able to perform his sadhana just by adhering to any of the paths shown in such books. We can also follow the same. As a result of intense practice such persons may attain siddhis (special faculties) and may thus become powerful. However, their Ego will be intact and not destroyed. Maya would not have left them. They will be attracted by name and fame! Therefore, they may declare themselves to be God – an incarnation of God. Many would believe and accept them as such! Since they would possess wonderful siddhis the world would be astounded by them. But, they will never attain Jnăna (True knowledge).
There lived in the past the great Kăvyakanta Ganapati Muni. He was an adept in Sanskrit, Poetics, Astrology, Vedas, Vedanta and Shastra. Moreover, he possessed great siddhis. He used to meditate for long hours without any awareness of his own body. He performed intense tapas. He has written many a wonderful Grantăs. He had a large following. But, such a great man sought Sri Ramana Bhagavan to attain Jnăna! This is because he had realized that all the qualities that he possessed were no sign of Jnăna!
Therefore, only if we perform sadhana with the help of a Guru can we attain Jnăna. If we take a look into our Epics we find that even asurăs like Hiranyakashipu and Padmăsura had performed intense tapas. This shows that anyone who has a strong mind can do tapas. But, Jnăna can be attained only with the help of a Guru.
Please check these: Excerpts from a discourse by our Sri Sri Muralidhara Swamiji