The essential idea of a brain wallet is that you commit your passphrase — the one used to create your private key — to memory. This means that no-one can steal it from you and therefore guarantees the security of your cryptos. It does of course suffer from the obvious drawback that if you forget the passphrase, you won't be able to access your cryptos.
It is a given that the longer the passphrase the more secure it is. It is also a given that the greater the entropy (i.e. the randomness of each individual word) the more secure it is.
In the past, some people have used phrases taken from well-known books as their passphrase. However, this is an incredibly insecure method. There are bots out there armed with hundreds of thousands of public keys created from phrases sourced from books. As soon as your public wallet receives cryptos, these bots will identify that the address is sourced from one of these phrases and you will instantly lose your funds.
For a real-life experiment, see: https://blog.bitmex.com/call-me-ishmael/
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