Do you remember when it started? I will start counting things sometimes when I'm bored and "waiting" like in a doctor's office I'll try to count how many holes are in a single ceiling tile and then multiply it by how ever many tiles there are, etc.

Is it particular type books/words? I use to have the compulsion to think/almost say, "Guess what we did in French today?" left over from French 1 in 9th grade, 1964ish! And, we had never "done" anything in French, LOL. I finally figured out it had to do with when I was anxious because I wanted to start a conversation with someone but didn't know what to say or how to say it; it was just my head trying to get me into communicating when I didn't know how. Lots of therapy and getting less anxious and learning better how to talk to others got rid of that particular urge/phrase popping into my head.

When I'm reading something that makes me anxious (or makes me think, personally, reminds me of something, etc.) I will often suddenly find myself in front of the refrigerator! It's like a trigger and my self tries to give me space/a break from the trigger. Maybe you start counting words because what you're doing/thinking is too difficult for you to face at the moment?