How do I open several files at once in Emacs?

You could try setting up a fileset in Emacs to accomplish this .

When you open emacs you can run dired. This will allow you to mark several files and open them at once. Not the best method, just putting it out there.

Dired Manual | Marking files with Dired

Yet another alternative is to just use a file glob (C-x C-f ~/foo/*.c RET, or proj/*/*.[ch], etc)

Have a look at the desktop commands, you can save your desktop into a file, say at the top of your project and then use desktop-change-dir to that project and it will load the files.

Have a look at the documentation on saving sessions. http://www.gnu.org/software/emacs/manual/html_node/emacs/Saving-Emacs-Sessions.html#Saving-Emacs-Sessions