Like the Tardis, it is bigger on the inside. Not only do you get to discover more adventures of the Doctor, but it also gives your more insight about the Doctor, Rose, Captain Jack, some enemies, etc. Not all stories are of equal quality, but the overall level is very good. And what is more, it is in paperback, so easy to take with you anywhere.
I bought this bookset because I missed the Rose-Doctor (especially 9) duo, and here I have 10 more adventures with them. I found it rather cool that the books are in continuity with the TV episodes. After seeing the two first seasons, you catch some little fun stuff in the books you had maybe missed before. And even though not all stories were as good as the others (I'm not fond of the Slitheen, and there's a whole book dedicated to them), I still enjoyed reading them for the Rose-Doctor chemistry. So if you like Doctor Who and especially 9,10 and Rose, you must have this in your book collection.