I've built 6 spruce spars in the past 40+ years. For 48ft box section
(hollow) to a 33 ft solid for my gaff rigged yawl. I learned at the side of
an old (to me) boat builder, in his back yard. I realize that opportunity
may no longer be available since wood spars have fallen out of favor and
spar quality Sitka Spruce is nearly impossible to find.

To answer your question, I have or have looked through just about ever old
school boat building books and can't remember any that actually gave step by
step procedures for building spars. Skene's Elements of Yacht Design gives
some details on mast section and material thickness, etc. but nothing on the
process.

Needless to say a solid spar from a solid timber would be easiest (if you
could find a good timber). However your want to build the mast type that
your boat is intended to have.

Perhaps you will find what you need at one of the few remaining wood boat
building schools.. Or the Wood Boat Societys. The one in Port Townsend, WA
has a library of books for loan, however you have to joint that group.

Steve
s/v Good Intentions