It's not a pressed flower. Look at the illustration; it's 3-dimensional, not 2-dimensional.
http://www.mangago.me/read-manga/aibiki/an/aibiki-chapter-0.html/4/
This would be representing them getting together after the story - which makes it confusing that it was used as the first picture instead of at the very end.
Also, get over yourself - it's not like this is a completely straight-forward story; it's easy to misunderstand. I am fairly sure (though not 100%) that Takechi saw the same book that Seichirou had noticed on the bookshelf as he was leaving (which is why the bookshelf was open). Seichirou seems to have remembered its significance earlier than Takechi ( http://www.mangago.me/read-manga/aibiki/an/aibiki-chapter-0.html/9/). It was only when Takechi removed the book from the shelf and the original love letter that they had both once found inside fell out that he thought to head over to their old rendezvous point to look for Seichirou.
Where the hell have all of you got your eyes. There is nothing written in this paper, but a dry pressed flower from that hill they used to meet each other. It says nothing to see if he can remember these old days just by looking at this book and the flower.
This is shown in the very first picture of this novel shot!