If you want to reach level 100 in a skill easily (and get a few levels while doing so), I'd recommend smithing. Simply find/buy (I'd recommend buying) all the iron ingots/ore you can, along with leather and leather strips. Warmaiden's is a good place, since the wife outside and husband inside both sell these.

Then simply make iron daggers. Once you run out of iron, switch over to leather bracers. Then sell all the items you've made (can even level up speechcraft here during selling). Simply wait until the shops reset (24 hours?), and repeat. You'll get to 100 in no time.

Also, a small note for the skill books achievement. All skills books will have a value above 50 gold (although not all books with a value above 50 will be skill books - I believe there's one worth 200 which doesn't raise a skill).