The boat rocks gently on the rippling waters of the Ganges. Rajesh Khanna, with a glass in hand, looks at Sharmila Tagore as a silhouette of the Howrah Bridge looms behind them. Chingari koi bhadkey, he sings, as the notes play with the waves.

For most viewers, the song from the 1978 film Amar Prem has been a symbol of the Calcutta landmark - the iconic Howrah Bridge. The director of the film, Shakti Samanta, wanted a boat song for his film - and the bridge as the backdrop.

"[But] the song was shot in a boat rocking on a meadow in Natraj Studios, Bombay, and later montaged with the Calcutta skyline against the Howrah Bridge as the backdrop," says a new book that traces the history of songs.

The book Gaata Rahe Mera Dil by two music lovers - Anirudha Bhattacharjee and Balaji Vittal - tells the untold stories of Bollywood music with a list of 50 classic Hindi film songs.

"We have tried to capture the history of each song," Bangalore-based engineer Balaji Vittal says. "But we didn't want to make it a technical discourse."

For Hindi film and music lovers, the two gathered the stories that played behind the songs. For instance, few would have known that director Dev Anand had initially approached S.D. Burman to compose the music for his 1971 film Hare Rama Hare Krishna.

The authors write that the film - with hippies smoking pot - was too "trippy" for Burman senior, and the film's music was finally composed by his son, R.D. Burman.

Dev Anand's first draft of the film had Jasbir (Zeenat Aman) falling in love with Prashant (Dev Anand), not knowing that he was her brother. "It was only after SD's insistence that this was dropped. According to Dev Anand, Pancham [R.D. Burman] created the tunes in two weeks flat."

Burman senior knew then that it was time for his son to take over his mantle. "SD used to take morning walks in Juhu where people upon recognising him, would say, 'Look, that's S.D. Burman'. One day, just after the release of Hare Rama Hare Krishna, he told RD, 'Today people recognised me, not as S.D. Burman, but as R.D. Burman's father.' That's how the Navketan baton was passed on from father to son."

The book is a part of the "50-series" of the publisher, HarperCollins. When Bhattacharjee and Vittal were approached, they were writing their first book, The Man, The Music about R.D. Burman. The subject interested them, and soon they were on board.

The two began work in 2009. The toughest job of all, they say, was in shortlisting 50 songs. They first listed 100 songs, which they brought down to 75, then to 60 and finally to 50. "But we knew that the list should be representative of various genres, songs, composers, filmmakers and even decades. At the same time, the song should have lived on for at least 20 years in the memory of the people," Calcutta-based Anirudha Bhattacharjee, who works with an IT company, says.

This book, which opens with a chapter about the contribution of K.L. Saigal to Hindi film music, covers a wide spectrum of Hindi music - from Chale pawan ki chal from the film Doctor (1941) to Aye ajnabee tu bhi kabhi awaaz de kahin se from Dil Se (1998).

The authors consciously tried not to pick up the most "obvious" songs. So instead of going with Awaara hoon - Raj Kapoor's hit song from Awaara (1951), they choose Tere bina aag yeh chandni and Ghar aya mera pardesi from the film.

Some of the songs - like Chingaari - were selected because of their spectacular picturisation. Some others were picked because of the genre they represent, such as Jaidev's Allah tero naam. The soulful Lata Mangeshkar song surprisingly never made it to the top of the charts when the film Hum Dono was released in 1961.

"But we kept it on our list because it is a bhajan, a different category of music," Vittal says. "Also, we have tried to explain how Lata Mangeshkar steps in to join the chorus and steps out effortlessly and subtly in the song."

Four songs of the Seventies' superstar Rajesh Khanna figure in the book. But the authors list just one of Amitabh Bachchan's many songs that became mega hits - Yeh kahan aa gaye hum (from Silsila, 1981).

"Rajesh Khanna dominated Bollywood from 1969 to 1973. All his movies had amazing music but music was never the USP of Bachchan's movies," Vittal says.

Through their list of 50, the authors trace the remarkable journey of Hindi film music, marked by changes in voice qualities, in the choice of instruments, in the style of singing and in recording techniques over the years.

The arrival of the disco era is celebrated with Pakistani singer Nazia Hassan's foot-tapping hit Aap jaisa koi (Qurbani, 1980). The authors recall in the book how composer Biddu auditioned the London-based singer and recorded the song in a London studio.

The song became such a hit that 40,000 people gathered at the Mumbai airport one day. "Prime Minister Indira Gandhi was arriving in the city. But the crowd had actually come to receive a 16-year-old whose flight had landed around the same time," the book states.

Those who believe that S.D. Burman was unfairly overlooked by Filmfare Awards may be touched by a little nugget that the authors share about the composer. His song Poochhon na kaise maine rain bitaayi, sung by Manna Dey for Meri Surat Teri Aankhen (1963), was one of Burman's favourite compositions.

"During the announcement of the Filmfare Award nominations for 1963, SD was hospitalised after an eye operation. When Manna went to meet him at the hospital and conveyed to him the news that the song had not been nominated, tears rolled down SD's cheeks from under his eye patch."

Lyricists have their stories to tell too. There is a real-life tale behind Indivar's Kasme Vaade pyaar wafaa from the film Upkar (1967). According to Anandji of the Kalyanji-Anandji composer duo, a young banker employed with Barclays Bank in Tanzania had fallen in love with a girl in India. He gave her a 25-paise coin as a token of promise that he would return to marry her.

"I was on a tour to Africa and the young man asked me to convey to her his intent. When he returned to India, we received the girl's wedding card. Indivar and I were in a car, wondering how to break the news to our friend, how much it would hurt him when he would hear the news of the girl's marriage. I had fractured my leg and was in a slightly philosophical mood, probably because of the injury. Suddenly, I said, ' Yeh kasme vaade pyaar wafaa, sab baatein hain...' Indivar said the lines could make a mukhra. We reached my house and created the entire mukhra."

Vittal and Bhattacharjee's ode to music is a treasure trove of memories. "It is for Bollywood lovers who have been intrigued by Bollywood's music for generations," Vittal stresses.