I am hoping that with the vast amount of reading experience, someone may be able to help me find this book.

I don't remember the name of the book, series, or the author, so here is some of what I do remember, in hopes that you can help me.

Main character is called "Fitz" or Fitzgerald.
He lives in a trailer, some kids shoot his cat and when he is burying it he finds a cement slab. After excavating the slab he finds stairs leading into a room, he falls into one of the walls and ends up on a beach with a wrecked ship.

Inside of that ship he finds gold coins, at which point he comes back to "our world" and contacts a friend to sell them.

Some other bits that I can remember, people from the other world have some kind of mental powers beyond ours.

During the main characters trips into the "other world" he comes across a blue<?> panther that used to be a jazz player?

He also comes across a platoon or so of < German ?> military from WWII.

The book cover, if I remember correctly, had a motorcycle on its side with a man holding a rifle and a large creature coming over the top of a sand dune.

Sorry it isn't a whole lot to go off of, I've been searching for about a year for this book, my understanding is the author has died and the series was never finished, but I could be mistaken.