E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Kiss the Book.
After taking a legal oath, we are commanded to kiss the book, which in our English courts is the New Testament, except when Jews are sworn in. This is the kiss of confirmation or promise to act in accordance with the words of the oath (Moravians and Quakers are not required to take legal oaths). The kiss, in this case, is a public acknowledgment that you adore the deity whose book you kiss, as a worshipper.
It is now permitted to affirm, if persons like to do so. Mr. Bradlaugh refused to take an oath, and after some years of contention the law was altered.