<human>: I'm a 26 year old female and am quite attracted to a guy I work with (he's early 30s). He can be quite quiet though we've been talking more recently. We have some common interests and I have felt a bit of a spark when talking to him. I decided to try to get to know him better and sent him a couple of casual, chatty IMs at work. He was friendly and responsive and kept the conversation going, asked me for book recommendations for his holiday. I added him on Facebook and sent a follow up message a few days later, just briefly suggesting another book I'd just finished. He got back to me and said he loved that author and asked if I'd heard said author's radio show. We sent a couple more messages, but he never replied to the last one I sent, which was a little longer though pretty open-ended. I don't get the sense that he's a big Facebook user. Yesterday was the first time I've seen him at work since any of these conversations took place. He knocked on my open door to ask a question (I don't think he went out of his way to find me - my door was open and I was sitting closest to the front door of the office, and it's a question he needed to ask someone). He didn't seem as friendly as I would've expected - he didn't make any conversation, just pretty much got the info he needed then left. Today I spoke to him a little while in a group of people and kind of felt like he wasn't making eye contact with me or speaking to me directly. 

If he hadn't been responsive to my chatting to him, I would just assume he had no interest in me and give up, but I'm a little bit puzzled. I also don't think I've made any romantic interest so blatantly obvious that he would have any reason to be stand-offish with me if he just wasn't interested. Actually, I find it really hard to let anyone know I like them and suspect I'm quite hard to read. 

Any thoughts on what his behaviour might mean and what I should do, if anything? 

Thanks!

Can you condense the text into a few key points?
<bot>: struck up conversation with a colleague via IM and Facebook and he seemed receptive but now seems a little stand-offish in person. Not interested?