Someone who knew of my love of Douglas Adams and Terry Pratchett recently recommended Tom Holt to me. On their advice, I picked up Falling Sideways, and am currently about 100 pages into it.

So far, while the book is not bad, it isn't charming me or making me laugh out loud, or even smile wistfully. Considering that first impressions are everything, is there a Tom Holt book that I should have started out with? Somthing a bit funnier, perhaps?

I haven't read Tom Holt yet, but I've had him recommended to me in the same way, and my husband is just now reading his first Holt, "Expecting Someone Taller." My impression is that he's just not quite as good as Adams or Pratchett, though still fun. And "Expecting Someone Taller" was recommended as one of his best ones (I think it may also be his first), so you could try that.

Also, in a similar vein, I would recommend Christopher Moore ("Practical Demonkeeping"). Him I have read, and I like.

I enjoy Tom Holt quite a bit.  My favorites are The Portable Door, and You Don't Have to be Evil To Work Here, But it Helps.  I haven't read Falling Sideways yet (it is on my WL), but I have read almost all of his omnibus editions.  Some are more clever than others, but all are enjoyable in a dry wit kind of way. 

And Christopher Moore rocks!