I have seen plans that are for purchasers use only, to build only one model.
So the whole area is a little unclear.
Basically, if you copy anything without the copyright owners consent you are probably infringing copyright.
This is what most people do.
The fact that you can go to a library and copy some things is not an exception to this. There are specific industry accepted requirements and limits on copying of this type of material. Just have a look at the notice on the side of the copier.
Copyright, (and parents) are areas where the whole underlying philosophy is under pressure to change due to modern technology.
Originally, these concepts were seen as a bargain between creative individuals and society, whereby someone who created an intellectual concept could be given a limited time to benefit from it before it became incorporated into general use in society. Originally, this time was very limited - a matter of a few years.
A whole industry started up around this, and every time copyright started to run out they pushed to increase this time, so now we effectively have unlimited copyright spans, and copyrights owned by commercial operations which never die. The original creators never benefitted much from this change, but lawyers and commerce did. They created a new idea - that of intellectual 'property'.
I happen to think that this is an evil concept, and appallingly bad for the improvement of humanity (but then I would - my formal training is in philosophy...). Others have thought so as well - notably Richard Stallman. Commerce keeps waving the pretence that this is about giving creators their due reward, and pretends that 'unauthorised copying is theft' - both of which statements I deem to be untrue.
Modern technology has thrown a huge spanner into the works - the ability to copy information items for no cost. It is a standard law in economics that, in a perfect market, the price of a commodity converges on the Marginal Cost - the increase in total cost as a result of producing one extra unit. The cost of copying a digital item nowadays is £0. So even in theory, the value of information items is non-existent.
Industry, of course, has a different view, and is trying all sorts of tricks to make people pay as much as they can get. In response, Richard Stallman developed the concept of 'copyleft', a kind of copyright agreement which specifically requires people to copy intellectual property for free. Linux, and much other 'free software' is released under this agreement, as are the plans on my site - it's formally called the 'GNU General Public Licence'. See
http://en.wikipedia.org/wiki/GNU_General_Public_License
