That or which?

[sophie_1003]

What are the rules surrounding the use of these two words; for example; which is correct-- I decided to read the book which is about dogs
or
- I decided to read the book that is about dogs?

[MarkRae]

Not really. The first sentence is always correct.

The second sentence could be correct if it were a bit longer, e.g.

That's the book, which I was reading yesterday, that I think you'd really enjoy.

[swedeheart]

But is your sentence No. 2 not comparable to this example http://i54.tinypic.com/2l8w3zo.png ?

[sophie_1003]

Think that would only be correct if you'd previously spoken to the person about the book swede, so for example-
Tuesday-- Me- (Through text to friend) I'm reading a really good book about cats at the moment
Wednesday-- Friend comes round to house and spots book on table, Me- That's the book, which I was reading yesterday.
But even then it doesn't sound right so I may be wrong!

[swedeheart]

I wouldn't have thought so, but I see what you mean, sophie. Wish "my English ears" were as good as my Swedish ones - it's very frustrating for me not to feel certain what's right and what's wrong;)