Here is a collection of quotes from Srila Prabhupada about the importance of book distribution.

Actually producing and distributing books is our most important engagement, all other engagements culminate in this one end — distribution of books. (Letter to: Tamala Krsna  —  Bhaktivedanta Manor 27 July, 1973)

There is no doubt about it, to distribute books is our most important activity. The temple is a place not for eating and sleeping, but as a base from which we send out our soldiers to fight with maya. Fight with maya means to drop thousand and millions of books into the lap of the conditioned souls. Just like during war time the Bombs are raining from the sky like anything. (Letter to: Ramesvara  —  Bhaktivedanta Manor 3 August, 1973)

Our main business is to distribute my books and people will come gradually. Everything should be done for that purpose. (Letter to: Ramesvara 7 November, 1976)

This [book distribution] is our most important work. All the leaders should tax their brains for increasing the sales of our books. I have always said that if we simply rely on book distribution all our needs will be met. (Letter to: Rupanuga 11 January, 1976)

Anywhere, this is our main program. Print books as many as possible and distribute. This is our main program. All other programs are secondary. So with this aim in view, work all together. (Room Conversation — January 9, 1977, Bombay)

Regarding printing our books and literatures, I may inform you in this connection that I saw one bulletin of “Indian Railways” in which it was specifically advised that every railway servant should see to it that the wheels of the carriages or vehicles must be moving always, which means that the railway is going nicely. Similarly all of us should see that our literatures are profusely distributed. That means that our missionary work is going on nicely. Otherwise we are simply sleeping and eating. (Letter to: Karandhara  —  Nairobi 9 October, 1971)

Ramesvara: Everyone is now wearing these “Double It” buttons. The whole movement is simply thinking of doubling book distribution, doubling it.
Prabhupada: Yes, that is our real mission. Yare dekha tare kaha ‘krsna’-upadesa. That’s all. (Discussions — June 2, 1977, Vrndavana)

The main thing is to distribute books more and more. That remark by the man in Houston is to your credit, that this movement is becoming an epidemic. Actually, everywhere envious people are against this movement, especially communists, because this movement is a threat to them. The main thing is to distribute books. (Letter to: Balavanta  —  Vrindaban 4 October, 1976)

I am glad to know that your sales of our literature and books are very good and improving more. It is natural that such distribution of literatures should increase as the people hear more about Krsna Consciousness Movement, so distribution of our books and literatures is our major propaganda program. (Letter to: Ranadhir, 29 June, 1970)

We don’t approve that go in a solitary place and chanting. My Guru maharaja did not approve! This is all nonsense. You cannot chant. Sir you’ll sleep only and think of all nonsense. You must be engaged for preaching work! (Morning walk LA 1973/12/14)

So-called transcendental meditation, go to the Himalaya and go to the forest. We are not interested in all this nonsense. Our only business is to spread Bhagavad-Gita As It Is, door to door, town to town, city to city. We are not going to seclusion. Prahlada Maharaja said, “This is professional bluff: ‘I am going to the Himalaya. I am going to the forest.’ ” Prahlada Maharaja condemns that “These are professional bluffs.” Or they may be sincere, but still, they are trying for their own salvation. Prahlada Maharaja said, “I am not interested in that sal… I am interested for everyone’s salvation. Everyone must go back to home, back to…” That is Vaisnava, not that “For my own salvation I go to Himalaya or in the forest and transcendental meditation, nonsense…” We are not interested in those things. (laughs) (Morning Walk – February 3, 1976, Mayapura)

I am so much pleased to hear how you are so enthusiastic for distributing our books and literatures. Krishna will surely bless you. This book distribution program is our real propaganda work and Krishna is giving you intelligence how to do it nicely. Thank you very much. (Letter to: Madhucara — Nairobi 25 September, 1971)

I am very glad to hear how the book distribution is increasing more and more. This is our greatest weapon. The more the books are distributed, the more the ignorance of the Age of Kali will be smashed. The world is feeling the weight of this Hare Krishna Movement, especially in your country. We have to increase this book distribution work more and more to firmly establish this Movement, which is the only hope for the suffering living entities. (Letter to: Balavanta  —  Vrindaban 23 November, 1976)