One thing that has helped me is understanding what "one" means. The world eschad in Hebrew means a one of unity, not one in pure singularity. Jesus said that he and the Father are 'one'. He also said that we would be one with him and the Father.
In marriage, two distinct people are united and called 'one' by God in Genesis. If you have been married for a long time (a good marriage,preferably!) the meaning of this becomes easier to grasp.
Obviously, the physical unity of two bodies operating together as one. A lock and a key, a violin and a bow, are often considered 'one'.
Also, we would count a woman just as 'one'... but if she is pregnant, she is technically 'two' in many people's view.
One of my favorite verses comes from Isaiah 59:15-16
The LORD looked and was displeased that there was no justice.
He saw that there was no one, he was appalled that there was no one to intervene;
so his own arm [Christ] worked salvation for him, and his own righteousness [HS] sustained him.I think of Christ as the arm of God, doing everything God wishes, empowered by the Spirit.
Another analogy is the sun (Father) and the rays of light (Son) and the warmth of the sun (HS). You can consider them individually, but you can see how they are all together 'one'. The Father, Son, and Spirit share many definitions and actions; the lines that define their 'edges' get very blurry to me.
The trinity is not an easy doctrine, though I believe it is correct. However, if the trinity was of utmost importance to understand perfectly and believe in for salvation, you would find the statement several times in the NT. Instead, one only finds believing/trusting in Christ repeatedly mentioned as utmost importance. Thankfully, becoming a theologian was never a condition for salvation. (Hey, just look at me! )
I imagine that book might have some arguments and explanations you've never considered, so it might be interesting for you to read.
Just remember, you can take your time and explore this issue; you can't force yourself to believe something you don't. If you someday see how it is acceptable to believe, believe it then.
bebu