Why does the Giver have thousands of books, while others in the community have only three in Lois Lowry's The Giver?

The Giver is the keeper of memories and source of information for the entire community; therefore, only he needs the records, resources, and learning found in books.

In fact, the knowledge that the Giver obtains from the books is kept from others in the community in order to maintain peace and contentment. In the dystopia in which Jonas, his family, and others live, many memories have been eliminated because they can cause discomfort, discontent, and...

The Giver is the keeper of memories and source of information for the entire community; therefore, only he needs the records, resources, and learning found in books.

In fact, the knowledge that the Giver obtains from the books is kept from others in the community in order to maintain peace and contentment. In the dystopia in which Jonas, his family, and others live, many memories have been eliminated because they can cause discomfort, discontent, and even strife. The Giver is the keeper of these memories, which is why he is normally called the Receiver of Memory. However, he is currently acting as the Giver by passing on these memories to Jonas, his apprentice receiver.

Books contain the history of people's thoughts, actions, and conflicts. Such history has been eliminated from the knowledge of the members of the community because it often a history of wars. Moreover, new ideas often effect changes, and the society in which Lowry's characters dwell is one that strives for Sameness and "precision of language" in the hope of eliminating conflicts. This society is also a closed one constructed so that everyone will remain safe physically and mentally. In such an effort, many memories of the past are gone to all but the Receiver of Memories.
If ever there is need for knowledge outside the ken of the Elders, the Receiver of Memory is called upon to give them knowledge that may help them from his memories. In Chapter 10 the Giver tells Jonas,

"....There's all that goes beyond--all that is Elsewhere--and all that goes back, and back, and back. I received all of those, when I was selected. And here in this room, all alone, I re-experience them again and again. It is how wisdom comes. And how we shape our future." 

The Giver adds that he feels greatly weighted by all these memories before transmitting them to Jonas.