Book 58: "Downs-Lord Dawn" by John Whitbourn

[kittiwake]

Thomas Blades, a seventeenth-century English curate living in the North Downs, finds a portal to another world in the grandfather clock that he inherits from his father. He finds a landscape that matches the geography of the North Downs, but seems completely empty, without houses, roads or agriculture. There are humans there, but living in primitive conditions in underground burrow complexes, as the savage eight feet tall purple humanoids known as the Null are the top predator, hunting humans for meat and tearing them apart when they catch them. Blades rationalises the presence of the Null, saying that they must be biblical giants who never made it into Noah's Ark in our world

He resolves to help the humans fight back against the Null, with the help of seventeenth-century weaponry. Gradually the humans build an above ground society, helped by slaves from our world, who are kidnapped to order for their skills/
Blades becomes their Emperor, the Downs-Lord, but his many wives and children are jockeying for power and his is not the only portal into this world.

I hadn't heard of this author before selecting this book for a ReadItSwapIt exchange, but I enjoyed it and will try to get hold of the sequels. I prefer alternate history stories that don't include fantasy elements, and "Downs-Lord Dawn" just made it, as the Null seemed more like aliens or the result of alternate evolution than like fantasy creatures.