Mantra is not a secret. Initiation into the mantra (mantrōpadesha) is a secret. Even if you read a mantra in a book, if it has not been initiated to you per proper procedure then you cannot call it a mantra. In books, what you read is only for knowledge but not to be used for spiritual practices.

Vernacular language has only 50 alphabets. Any combinations can be made with these 50 alphabets. Just because you can recognize and read alphabets does not mean that you can understand the mantras. Those energized words that come from a Guru and which you accept can only be called a mantra.

(Glimpses of Bhaktimala)