In a similar Format of The_Loney_Goatherd
Fictional
I'd say my favourite fictional book that I've read so far would have to be Malazan Book of the Fallen: Memories of Ice. An absolute masterpiece. The whole series is great (1/3 of the way through the eigth book) but there's something magical about the third one.
It's themes are astounding. It's world building is fantastic and he might be the master at making you like minor characters in just their introduction and kill them of in the same paragraph. You thought G.R.R.M liked killing off people...
Spiritual
My favourite spiritual book would have to be Romans or Acts. Though I think the gospels are of course more important in spreading the message, I find Romans sometimes personal compared to Mark, Matthew, Luke or John. I'm also fanciated by Acts because it gives us the opportunity to see into the early church and it's major struggles and to see my second favourite person in the New Testament for the first time. Paul. Maybe one of the most interesting people in Christianity's history.
If we're talking Non-Canon, I'd say it would have to be
Abide: Understanding the secrets of living for Jesus by Warren.W.Wiersbe.