Benefits of involvement in the Open Source movement

I had an email from an independent publisher recently who found some of my work online and apparently it impressed her.

I am currently seeking a co-author (we have one author in place for this book already), and was very interested to look at your involvement with IB Maths, and related websites, and also your approach towards making maths relevant to the student - this is so important with Maths.

At this stage, if you are interested in being considered as the co-author on this book, we would need to see a sample of your written work, ideally material aimed at Maths. Would this be possible? It need only be about 3 or 4 pages, but ideally will show how you would introduce the teaching of a topic, some worked examples and then a few practice questions. This give us an opportunity to assess your writing style, how you use context etc. Let me know how this sounds.

Obviously I haven't got the job yet, I still need to prove myself a bit, but without my involvement over the past year or so with Open Planner and some other websites I've designed, I would never have gotten this offer. So this means that the work I've done, unpaid, in my spare time, as seen benefits in my professional life as well. I'm extremely pleased to have been offered this opportunity, and now we'll see if I can follow through and produce the document this publisher wants.