I found Ron Holleran's book "Nostalgia" and John W. Lawson's book "Flathead Facts" to be very informative.
Ron draws on his years of experience building street and race flatheads and in a straight forward writing style describes the build process. He explains the changes in the engine through the years, what works together, what don't, what processes are necessary, which are superfluous.
John starts with a basic flathead "mule" engine and as he makes changes and modifications, dyno tests the engine to see if there's gains, or in some cases, losses for each change.
Each of these books could save time and money because of the trials these gentlemen have put the flathead engine through.
Check the Swap Meet for availability.
Howard