Translation:That book which is beside the new tv is long.
Probably because nobody would say it that way in English. Emphasizing the length we would say as noted above (The book which is next to the new television is long). To emphasize the location, we would say "The long book is beside the new television" or "The book that/which is long is beside the new television."
The contributors want to show, how subclauses have to be built. So far it is OK to show the analogy of the english language. But the contributers want to do that at a very early point of the course. The sentence above is part of the lesson "Choices 1".
In other courses like French for Germans or English for Germans you don't get plagued with something like this in the first 75% of the course and not in that amount at all.
The slight difference is between "that book which is..." (as opposed to any other book) and "that book that is" (describing in abstract). "Which is" is "amelyik"; "that is" is "ami".
Regarding "az a könyv", I think it's been fairly consistent. "az" means "the" when preceding a word beginning with a noun (eg "az elefant") and means "that" in other cases. Because "that" is not an article, we need the definite article "a" before the noun as well, hence "az a könyv" = "that book". You'll also find "that elephant" is written "az az elefant"; the first "az" means "that" and the second "az" is the definite article.