Oh! Yeah! I played all three Discworld games, but actually, I needed walkthrough help for the first two, puzzles were completely illogical as you say. I imagine that's part of the charm of those games :P. Discworld 2 being actually more enjoyable than the first in my opinion.
About Sherlock Holmes games, there are a ton out there, from the eighties to nowadays, but to tell you the truth, I don't really like those from Frogwares, if I can recommend Holmes games these are the ones:
Sherlock Holmes - Case of the Serrated Scalpel
Sherlock Holmes - Case of The Rose Tattoo
Both are from the same studio, Mythos Software. And as Sherlock fan as I am (I hope so, I read most of the books twice and watched the Granada TV series like four times, lol), these games are actually extremely well inspired, especially the second one. I wonder if you ever tried them.Cheers!