Angels and Demons is the first book in the highly popular Robert Langdon series. The book is of mystery-thriller genre, written in first person using points of view of multiple characters, but the protagonist is Robert Langdon. The book starts with a murder and then follows the characters' journey to find the killer and avoid a catastrophe.
Dan Brown, who himself worked as a teacher, based part of the character of Robert Langdon on himself. He worked the plot around the age-old debate of religion versus science with a few other subplots around it. The subject, already controversial, leads to the tension in the plot.
The book starts with a murder of a scientist at the CERN. Shocked at having found a cross in the scientist's apartment, that he finds it unbelievable that the scientist was a priest too. Ideas like cross-connections between people, voice of God and even presence of God are debated and tried to explain scientifically.
Most characters are in a fix based on their loyalties. People like Ventresca find it horrifying that a priest and nun could have a love affair even when no physical relation was involved. People like Kohler condemn science as he was denied medical aid as 'Will of God'. Science is accused of trivializing universal matters in terms of equations and numbers and religion is accused of controlling the potential of humankind.
The title and the ambigrams associated reflect this too. It is never clear that between religion and science, which is angel and which is demon. This question is intensified with the use of ambigrams. An ambigram appear same in both directions. The title on the cover page thus reflects this.
The plot also incorporates myths around ancient scientists and their demise fueled by religion. So, it's almost ironical that bishops are killed by men of science in the same fashion.The plot is basically an attack on religion by science, but is revealed to be vice-versa, thus proclaiming that the two can't sustain together.