Haha, yes I realised I had spelt it wrong after posting but didn't edit it.
You could put a level 80 restriction on the tomes, or you could only let lower level characters access the relevant level of recipe until they reached the required level (so <60 toons would only be able to access vanilla level recipes until they hit 60, when the TBC recipes will become available in their book) to solve the problem of level 1 characters having 450 professions.
Ultimately if you had enough characters you could have all professions maxed, as you can now, but if you decide that you would rather your hunter was the engineer, not your paladin, you would have a means to swap professions over without simply wiping them on both toons and powerlevling them back to 450, losing all rare and unique recipes you learnt the first time round.
The tomes could sell for something like 500g, or even a 1000g. Enough to be significant but much cheaper than powerlevelling two professions back to max. Perhaps if you don't have alts the tomes could even be BoE, so you could sell off your old professions if you wanted to change. I have max levelling enchanting on my paladin, want to change it, but I can't bear to throw away all the money I sank into it, all the rare drops I ran instances and grinded mob after mob for, and all the BoE and raid recipes I collected.
There's not anything else in WoW that requires you to destroy everything you worked for just to take a different path. If you change spec you still keep all your old gear and it only cost ~50g to go back. When you get a new sword you don't forget how to use axes.
It seems logical to me that you should be able to transfer your trade skills, they are after all a list of recipes, why couldn't you just put them all into a book and hand them to someone else?