The change to the second question begins in verse 23 because...
1) There are two questions about two different but related things. There has to be a transition from answering the first, to the second. It need only be identified.
2) Jesus begins a second warning, not a repetition of the first. This second warning is for them not to look for the Christ on Earth. If you notice, he's on Earth as he's giving this warning.
3) In Mark 13:21 Jesus is recorded as saying "Then, TOO, if anyone says to you "See! Here is the Christ..." making clear that he is referring to another time period when Christians will "too", or again hear of "false prophets". Not a repeat of the first "false prophets" warning but a fresh one. Following that, in Mark's account, verse24 goes on to say "But in THOSE days, after THAT tribulation..." showing that he is contrasting the first event, the fall of the Temple which he has just discussed, with "those" still far distant future days when "THAT" event of the "Son of man coming in clouds with great power and glory" will take place.
As far as a "book", I don't recall saying I wrote a book. Although with all that I have written so far, it is equal to a small book.
Revelation? Nobody knows this. We know parts, or believe we do. But in the end, without full knowledge, anybody who says he can explain any part of this book is on thin ice wihtout beaing able to explain it all.
And 'dem whatz tried, ...cried.