No. 46, Josephine Tey's The Franchise Affair, took a while to get through.
The mystery was good, but the book was written in the '40s in England, and it was laborious to read, taking me way too long to finish.
I was talking with a Spanish-teacher friend of mine about how reading a novel in Spanish takes so much more time because your brain has to work harder - it's not all automatic. I felt like that with this. Phrases and words were different, so my brain wasn't able to just cruise through it. Interesting. :)