Just thought I'd close the loop on this.
Thank you, Chris, for your reply.
There are a series of books on SP&S steam locomotives written by Ed Austin.
"Steam Locomotives of the Spokane, Portland, & Seattle Railway"
They are softcover, and each volume focuses on one or two classes of engine. They are around $20 - $25.
Volume 6 covers the 4-6-2 and 4-4-2 classes. All the books contain good reference photos.
These books are published by the SP&S Historical Society, so they do not have ISBN numbers,
but you can find them on-line if you search a bit.
(I dropped a hint to my son who asked what he could get me for my birthday, so hopefully,
a copy of volume 6 is coming my way
)