The Diamond Contessa. Keys to the Dimensions Book 8

Harry Blakey remembered a childhood secret - that there was a room under his folks' home which crossed into another world. When, finally as a war veteran, he came back to the old house, he investigated - and found his memory was true. There were indeed other Earths and other civilisations and adventures to be had - at great risks. For when he enlisted in the special commando corps organised to stop the interdimensional warfare, he came up against the terrifying hordes of the Diamond Contessa.
She had looted many Earths and her hunger was always increasing. No mere human heroics would wrest the keys of the world away from her - not white her army of monsters held a dozen civilisations in thrall!