I need to make a small website to be hosted at a domino server but as far as I understand, I couldn't use a simple .html file or .php, instead it works with some weird stuff like .nsf, right? Also, it isn't clear to me wich language is used to deploy webpages with domino server, and if there is a good book about it.
Starting with Lotus Domino 8.5.x you can create web applications using XPages. XPages "is based on web development languages and standards including JavaScript, Ajax, Java, the Dojo Toolkit, Server-side JavaScript and JavaServer Faces" (source: http://en.wikipedia.org/wiki/XPages).
If you are interested in learning more about XPages, the following book is very useful: Mastering XPages: A Step-by-Step Guide to XPages Application Development and the XSP Language
There is also the "classic" way of creating web applications with Lotus Domino if using XPages is not an option. The following book provides a good overview of that method: IBM Lotus Domino: Classic Web Application Development Techniques