It’s literally the third dedication in this book, and a heaping spoonful is just a really full spoonful, if you heaped it on so there’s a pile overlooking the edge of the spoon, about to fall out.

I have not looked for several years now and got a fifth car in my collection which is quite good looking.

The fifth I have released is sporty. Other cars besides the car are limos or trucks or family stationwagons. We don’t know; all we know is that the fifth car is sporty).

If this collection is to remain and have some future additions to its collection, he will add a fifth car to his collection.

Is these cars sporty or

sporty?

Is it an inside joke either person will say something or only the participants will know the meaning of.

a heaping third spoonful of gratitude

is cryptic, but the fact that it’s heaping he is trying to express not only very much but more than that

The author is thanking the support of two dear and steadfast friends.
What immediately comes to mind is had Coffee or Tea where the standard question is

one or two spoons of sugar
one or two lumps

This is implied with third and. They might have drunk coffee together as coding hours, during late night completion of the book (assuming this is from a book)

I think it is good to break sentences down into their parts. I can no longer resist the feeling of gratitude. It goes back and forth among me. This is two adjectives along with something called a “partitive”. The partitive is simple, its just a multi part expression containing a count noun and the word of. Two pieces on pie, bread a stack of bills. So the sentence adj adj partitive. What a great pie?