Book to read slow

I'm used to read, like, really big books in a short amount of time. I have 14 and i have read around 7 thousand books, most of them i read in a rush (Note: Didn't ruined my experience with the book). Also, keep in mind not all of them were so big. Some of them had 100 pages, and, most of them, had, like, 10 or 20 pages. Around 6000 of those 7000 had 20 to 200 pages, only 1000 of those books had 300 to, maximum, 3000 pages.
So, i'm trying to stop reading books just to add more of them to my list of read books, also, because it's starting to turn very expensive. I don't even have enough space in my bedroom for more books.

What is a book to me to start reading slowly. Read, think, read a little bit more and think even more?