Duke University's D. Moody Smith lucidly explains the theological ideas of the Gospel of John. Smith concentrates on its presentation of Jesus as the Christ along with such topics as God, the scriptures, the Church, and the spirit. An ideal introduction to the question of the origin of the Gospel of John as well as its theology.
In this brief introduction to the theology in John, Smith goes over what he argues are major thematical differences, touching on how they may have arisen. For instance, he points out the anachronism of referring to 'the Jews' and historicizes what likely the community which produced John was going through in light of passages referencing 'being put out of the synagogue'. Smith briefly touches on textual criticism issues and determines that John is unlikely to draw on the actual writings of the synoptic gospels, though obviously draws from a common well of traditions about Jesus, or Paul and highlights differences and similarities between these works.
I would have preferred a more systematic and detailed treatment here and felt that many of the arguments lacked sufficient support. I think however that has more to do with the length of the book and its objective of being a supplementary introduction than anything else.