This has to be The Devils of D-Day (1978) by Graham Masterton.
Quoting the blurb:
At the bridge of Le Vey in July 1944, thirteen black tanks smashed through the German lines in an unstoppable, all-destroying fury ride. Leaving hundreds of Hitler’s soldiers horribly dead.
Thirty-five years later, Dan McCook visited that area of Normandy on an investigation of the battle site. There he found a rusting tank by the roadside that was perfectly sealed, upon its turret a protective crucifix. Sceptical, he dared open it, releasing upon himself and the innocents who had helped him an unimaginable horror that led back to that black day in 1944. And re-opened the ages-old physical battle between the world and Evil Incarnate...
I found a review that quotes the warning you were mentioning:
All the devils and demons that appear in this book are legendary creatures of hell, and there is substantial recorded evidence for their existence. For that reason, it is probably inadvisable to attempt to conjure up any of them by repeating out loud the summons used in the text, which are also genuine. I would like to point out that the Pentagon and the British Ministry of Defense strenuously deny the events described here, but I leave it to you to draw your own conclusions. – from the Author’s Note
There are a lot more quotes and details at the linked review; I'm positive it's the book you're looking for.