In a book, there are two key parts that must be perfect. The first sentence, and the final sentence. Without these, well, good luck having your story read.
The first sentence of a book is your hook. It is what keeps the reader captivated for up to the whole of the novel. You can't have it be too long, or too short (unless you're Charles Dickens, but that's a different story). It can't be too descriptive, or too bland. The few words it needs has to set the story out straight from the beginning.
Most stories describe the setting. They throw you into an airport, a moving car, or a new house. From there, they go on to describe characters, histories, and settings. Action will happen and resolutions will be created. All in all, this is the sentence that gets you readers and tosses them out.
When the first sentence isn't done quite right, the story can start off on the wrong foot. Just yesterday I read a book, and something was just off until the author resolved it. This was because her first sentence didn't quite have the energy it needed. Luckily, she soon fixed it and I am now eagerly awaiting my next library trip for the sequel.
The final sentence in a novel is what keeps your readers coming back for more, even if it is the end of a series or a standalone novel. Let's look to the final sentence of the famed Harry Potter adventures.
"All was well."
This last sentence sends you on your way with a certain satisfaction, but still keeps you coming back for more and more. Right here I must say that J.K. Rowling is a genius and an artist.
Your last sentence is also very important because it can make or break your writings. With this last sentence you want to be able to sum up everything, without fully summing it up.
To say the least, the most basic parts of any writing can be the most important. It is suggested that you remember this well.