OED Historical Thesaurus published (22/10/2009)

Roget gives us the present day English vocabulary arranged by semantic fields. You start with a word, look up its semantic category (a nested hierarchy with labels like "1.2.23.14"), look that up and you will find your word together with all its near-synonyms or closely related (by meaning) words. The classic task for Roget is "I want a word that means something like 'insist', but a little different..." or "I don't want to keep on using 'insist' -- how about a word that means roughly the same, for variety?"
The OED Historical Thesaurus adds a historical dimension to this. Starting in the same way, you find a semantic point or field you're interested in, and it gives you all the words that have been used there, right back to Old English. You can see how the vocabulary for that particular area has changed over the centuries.
I have to admit: I bought myself a copy of OEDHT a couple of years ago, on impulse, for probably more money than was wise. It's two great hefty volumes, and it's on my bookshelf now, but I have used it very little.
I'm not at home with the semantic categories, and the historical information is extremely compressed, in small print with numerous abbreviations. Maybe I just need an online tutorial in how to use it.
Actually the whole thing is available online, alongside OED Online, which I use all the time; but if I learned to use the online thesaurus, I would realize I had wasted my money on the hard copy....
The sort of question which I have occasionally thought OEDHT might be able to answer is: I'm thinking about a modern English word which seems pretty basic, but I know it only goes back to the 17th century, or to Middle English. The concept is not new, so what words did they have for it before the one we know today?
Crystal says it's the first historical thesaurus for any language. The idea was suggested to the Philological Society by Michael Samuels in 1965, and 44 years later, there it was.