First, note that
every time a page begins with a certain letter, the next page will always begin with the same word. The first letter of that word will determine the first word on the next page.
There are 26 letters in the alphabet. Therefore,
If we look at the first words of pages, they must form a cycle of length 26 at most. This is the 40th page; therefore, we are well into at least the second such cycle.
The first word of the present page is "Till".
Because we are in the second (or subsequent) cycle, there must be a prior page that began with "Till". We know that the first word of the preceding page was not "Till" or the third and fourth word on this page would be the same. Therefore, we know that "till" is not the word corresponding to T. It must then be the word corresponding to I or L.
Consider the first word of the 39th page. Call it 39th.
Since we are so far into the cycle, we know that 39th must also be appearing for at least the second time. 39th must begin with either I or L. None of the seven letters on this page begins with I or L, therefore none of them is 39th. Because each word must contain the letter it corresponds to, that word must appear in its own expansion. Therefore, 39th must be more than seven letters long. Since 39th appears in its own expansion and it has occurred as the first word at least once before, we know that it occurs in some position after the seventh on every subsequent page including the 39th page. Therefore, the expansion of its first seven letters occurs later on this page.
For the bonus,
I got nothing, other than the above. Definitely, it is sufficient that none of the words in the sentence could be the first word on the preceding page but but I feel certain that there is something more intelligent to be said.