As he read, I fell in love the way you fall asleep: slowly, and then all at once.

This quote is describing the feeling of falling in love. It is saying that falling in love happens gradually, like falling asleep. Falling in love feels sudden, but it is actually the result of many subtle moments that occur over time. The quote suggests that falling in love is a process that happens gradually but eventually leads to the sudden realization that you are in love.