Lee McCormack’s $10 eBook has some good information in it, even if you’re not building a pump track (http://www.leelikesbikes.com/ebook-welcome-to-pump-track-nation)
Others have linked to the IMBA stuff which is OK. It’s all general guidance though so application on a particular site can need a bit of common sense / judgement of appropriateness.
Some new links to things I haven’t seen before too, thanks Asterix.
Ian Warby did a few articles / clips of trail maintenance on a website. Google should sort you out unless anyone else can link to them?
Regarding the ethos of unilaterally deciding to change a trail – it’s a tricky one. One man’s pleasure is another man’s poison. You have to be sensitive to this, within reason. Is there anyone else doing similar stuff or an established group already? It should go without saying that adding something which could catch people out is to be avoided, but you’d be surprised what such features might be (i.e. innocuous stuff). You have to be able to understand how a trail works / is likely to be ridden and how any changes might interact with that.
Getting involved with a local trail group is the best way to learn about this stuff. Only so much can be learnt from books, working with others who are more experienced is good.