Spinning Compass Network

Can someone please explain for me, what difference is there with using "the" or "a". I have not a direct translation for these words in my own language. I understand they go before certain nouns, to refer to them? But what difference is there if I say... for example, I am reading a book, or I am reading the book?

I'm think only a few people were aware that myself and Cleo had returned to the island. Things on arrival were a bit... hectic?

That said, now things are a little more settled and I've got some time to, I thought I'd say 'hello again' and share our news.

On the 31st August, upon arriving, Cleo went into labour (I don't know how many remember or knew she was pregnant when we were here last) and gave birth to a beautiful little girl. Both have been resting for a several days but they are both doing great.

We've named her after our grandmothers, so she is Evangeline Mary Dower.