I read the Gospel of John twenty times about this time last year. I either lost my notes, or failed to write them in a timely fashion. I had to go back through the book to collect my thoughts...
John's Gospel is very different from the other three. The language is much more abstract, and John doesn't spend much time on events (In 21 chapters, John advances to nearly the last week by chapter 11, possibly earlier). The arrest, crucifixion, and resurrection are compressed into the last four chapters.
If you want to know who Jesus is, and read just one book of the Bible, the Gospel of John is the book for you.
This book clearly spells out that Jesus is one with the Father, eternal, and the only way to salvation. Near the end, Thomas makes a clear proclamation that Jesus is God, and worthy of worship.
God is only as good as His Word, His promise. When it came time for Him to swear an oath, He swore on Himself. Jesus is that Word, and we can know the Father by knowing Jesus.