The book was a series where first a human enters the "Gnome" world and stop them from destroying the earth and the second was a gnome entering the "Human" world. I thought the title of one of the books was something like; "Human in the GnomeWorks", but Google doesn't help.
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Nick O'Donohoe wrote a pair of books called The Gnomewrench in the Dwarfworks (1997) and The Gnomewrench in the Peopleworks (2000).
The blurb for Dwarfworks:
Little people... with BIG problems. Autumn 1944. While the war rages, 4-F draft reject Grady Cavanaugh works a boring job as an engineer for a small industrial furnace manufacturer in New England. The he receives an order for a rather unique furnace - built to accommodate someone under three feet tall. Suspicious of the secrecy and odd accent of the buyer - Antony "Dutch" Van Der Woeden - Grady decides to investigate. What he discovers defies imagination...
The blurb for Peopleworks:
A common cause... uncommon allies. Summer, 1945. The war in Europe is winding down, but thanks to their alliance with the gnome-owned, dwarf-run Nieuw Amsterdam Metalworks, Plimstubb Industrial Furnaces still has all the business it can handle. Some of their clients might be a bit - odd - but work is work, and Grady Cavanaugh is happy to have a job.
But his greedy boss wants more - and he strikes a deal with Nieuw Amsterdam to provide gnomish technology to human projects.
He's forgotten one very important thing: Gnomes hate humans. And the machines they have provided are very, very dangerous.
Now Grady and his dwarf friends must find and disable them. There are lives at stake - even more than Grady can imagine...