But why would you give an opinion about "a specific problem in a specific jurisdiction?" That makes no sense. Either engineering science is applicable everywhere, or nowhere.A PE seal is often required for an engineer's "report" for a specific project in a jurisdiction. A letter addressing an opinion on a technical issue can constitute a "report". Nevertheless, this can be a gray area, as an outside consultant may offer advice to the EOR without being registered in that state. But in this case, a seal would likely provide added incentive for the EOR to accept the outside advice.The author of a textbook is not addressing a specific project in a specific jurisdiction.
I repeat: I do not think it either necessary or advisable to affix your seal to a letter commenting on a specific application over which you really have no control, if you're the EOR.
And if you're working for the EOR, then YOU are not sealing anything anyway, so why not just ask him what he wants you to do?