....recently read in one book that the spine of the blade and handle should be annealed.
I think you need to do some more reading. What book were you looking in?
The issue is that there is softer and there is soft
Softer than the cutting edge is alright. Soft as in annealled is not so good (on the spine), but not disasterous either.
Taking O-1 (ground flat stock) as an example, I believe, it is very poor practice to leave the spine and tang in the condition the steel leaves the factory. There are some makers who use a torch to harden just the cutting edge, O-1 shows a change, when etched, between any part that has reached critical temperature, vs areas that are still in a spheroidized condition. Such knives can be very tough, they will bend before breaking, but they can't hope to be as strong as a blade that has been fully hardened and then differentially tempered, or cycled then edge quenched. The annealled areas will have little strength.
Even a lamellar anneal will be stronger than spheroidized.
Having said that, the affor mentioned makers test and use their knives and find them satisfactory for use. Most people aren't going to use a knife hard enough to notice the difference in strength anyway.
In any knife the handle area is significantly reinforced by the handle material. Provided that the tang has been constructed properly there should be no reason for it to break even if it has been hardened to the same level as the cutting edge. Or bend if it has been left dead soft. The main danger area is where the blade exits the handle and anywhere that you have sudden changes of section, either in thickness or depth.
Wow, what a lot of hot air I have!
In the end, all that matters is that the knife does what YOU want it to do, or what your customer wants it to do. There are many ways of doing things. Other makers will have prefered methods which may have as much to do with the tools they have, or the speed they have to make knives, or the type of use their knives normally see. These things will not necessarily transfer to your own making. If YOU want to use the spine on a firesteel, harden away and know you are in good company with Alan Wood, Roger Harrington, and many production companies. If you think you want a tougher blade, go with an edge quench/spine draw, or torch hardening. You will then be in company with such people as Wayne Goddard, Ed Fowler, Ed Caffery, and Mick Wardell.
