The Ending is most important, except for the Start, and the Middle.

I've read that the ending of a book is the most important part.  Also that the beginning of a book is the most important part.  And of course, the middle has to hold up too.

Anyway, for me, the beginning is by far the most important part in the writing of the book.

But... I can see how the ending could be the most important part for the reader.  It's what he or she will take away, will remember.

But by the time I get halfway through a book, the course is set.  I'm playing out the story that was mandated by the original premise and by the character's reactions.

So it's easy for me not to put as much work into the second half as I do in the first half.  I have to consciously go in when I'm finished and work on just the second half, once I'm satisfied with the start.
I've already changed the ending quite a bit, cutting out some chapters that I decided were extra or unnecessary.  In fact, in several of my rewrites, I was conscious about working on the ending, even though I thought most of the problems with the book were at the beginning.

The most crucial and tricky chapter is about two/thirds of the way in, and I've worked on that chapter over and over again.   I doubt I'll ever construct a book again which depends on the main character trying to talk other characters into doing something, especially when what he's trying to talk them into sounds outlandish.

I at least upped the motivation for this scene -- which used to have a silly premise of PETA people taking up arms to fight the monsters who are taking their pets.  That was OK when the tone was different, but not after I made the tone serious.

The solution was actually easy -- that it is missing family members, and worse, children -- who are being taken by the monsters.  Not so hard to convince people that there is a need to take arms.  (Though the problem of convincing them that there is a supernatural cause is still there.)

So that's where I'm at right now.  I've got a couple more days, plus next week's writing session to concentrate on the ending, and somewhat the middle.

In my books, by the time I get to the second half of the book -- events start taking over, and it is much more action oriented, which for me is easier to write.  But I mustn't forget to make the character's reactions to the unfolding events real and meaningful.

I need an emotional entry point on the second half -- so I'll spend the morning looking for that.  The book works so much better when I can relate to the characters.  (duh.)