Translation:That book is boring.
Yes and no? "Kono" is close to the speaker and "sono" is close to the listener. More typically in English we would use "that" for an object close to the listener but not us, the speaker. I think "that" is being reserved for "ano" where the object in question is far from both speaker and listener. I will not argue that using "this" for both "kono" and "sono" isn't confusing though. It very much is. The Genki books teach "kono, sono, ano" as "this, that, that over there" to try to avoid such confusion and include diagrams about the near speaker or listener thing. Oh well.
I just got this sentence again and now it uses "the" for その (it used "this" when I wrote my long reply) and I would like to add that その is NEVER "the." If that is what made you ask if it should be "that," then omg yes, yes it should. I am so angry right now that this course is trying to equate その with "the."
Interesting, i tried to search some devide in the word つまらない. If there would be different meanings in it, then removingない, would leave the verb つまる, which amongst other things means to be packed, blocked, shorten andd.. to be a loss. So if this word was not conciously formed that way, it is at least an easy bridge to remember. Something which is not full or clugged (of fun stuff) or is not a loss, is boring!
Sorry, but you understood it wrong.. The last い shows it's an い-adjective. It also seems like a ~ない form, but this simply just has to be remembered. I'll show you why です doesn't show this (it's "just" a polite way of ending the sentence and shows the tense of the sentense):
かれはハンサムです。 He is handsome. ハンサム is a な adjective, but です is still used. Also: かれはハンサムじゃないです。He is not handsome.
So again, in all the examples です is being used, so it has an other meaning than showing what kind of adjective is used.