This is probably the most helpful book on beginner water color painting. Although the subjects are mostly limited to landscape, these core skills will help me in my own work past landscapes.

Cathy Johnson has published several fantastic illustrated journal books, all of which are worth finding.

This one on getting past the fear of this medium by practicing simple approaches will be very useful for me and for anybody who wants to learn a new medium, whether for illustrated journals, landscape, still life, or urban sketching.