Thank you very much people for all the great suggestions. There are a lot of interesting hints here. I've been thinking for a long time to start using the tickler file, maybe this could be a good 'excuse'.
I also find comfortable Jrdouce's idea about @agenda. It's kind of a 'list of lists', if I get it right. It would allow me to remember my commitment on a regular basis without being too overkill.
The thing with this friendship is that it is quite an old acquaintance, and one of those friendships that do not really require that much frequency. In fact, I think getting together more often could even be dangerous for our relation, it's like we need to go and do our lives for a while and 'let the battery recharge' before someone calls the other one again.
I am being that specific here just to note that I don't think Barb's 'do it right away' approach, while very praiseworthy usually, would work in this case. If, say, the day after one of our meetings, I'd send the book to my friend right away, I think it would not feel comfortable for any of us; but after a couple of months, bringing the book would be the perfect warm up to continue the conversation where we left it.
Thank you all guys.