Well, these were always going to be pretty obvious, weren't they? Another tie of supreme awesomeness.
Hitchhiker's Guide to the Galaxy, series, Douglas Adams
|This is the British cover,|
in case you haven't seen
Images from Goodreads.
I don't know why you'd need another favourite H book besides these, but I suppose there are a few non-fantasy people out there, so have at it in the comments.
Runners up: Why do so many great books start with H? Howl's Moving Castle, Diana Wynne Jones; The Hunger Games, series, Suzanne Collins. (Yes, I know Howl's is also a series, but I haven't read the rest yet. It got into the runners up on the strength of the first book.)