My guess is that it's the book of Leviticus.
Even readers who know the Holy Scriptures very well, will often just skim over Leviticus; newcomers are likely to have a very short relationship with the Bible if they start with this book.
On the surface, Leviticus deems downright boring and distasteful. The content is made up of complex rules for the worship of God's People at the time of Moses, with lurid details of how to use the blood of sacrificed animals by pouring, sprinkling and flinging (flinging!) it about. It includes a manual for priests in ancient Israel to determine whether a house has mould.
Perhaps the greatest challenge to appreciating the wisdom of Leviticus is that the book is really written in code, and the code is difficult for us to break without some painstaking work. The book is indeed filled with regulations for animal sacrifices in the ancient Jewish Temple and the like, but Leviticus is not about these things. Deep within its strange patterns and regulations is found a series of laws of social justice which teach that because we worship God we are commanded to protect the poor, the stranger and the vulnerable from cruelty and wrong.
Here again we may pause: Leviticus teaches that worship and service are one and the same thing.
You cannot serve God truly without caring for others. And this is summed up at the centre of the book, which Jesus later teaches is at the centre of our calling as Christians, and our life as human beings (Leviticus 19.18),
You shall love your neighbour as yourself: I am the Lord.
How can our worship of God in the Cathedral deepen in us love, and active service, for our neighbours?