Hey! I've been playing for 15 years...and I just have a few things to add.
For a really informative book on composing for the harp, I highly recommend looking up Dr. Ruth Inglefield's book: "Writing for the Pedal Harp".
There are many effects the harp can do, including one called a Thunder effect, which is basically just slapping your hand against the lowest bass strings. This makes a very loud clanging sound. You can also perform percussive effects on the soundboard, either with your hand or with various mallets---not to extremes! We still aren't drummers. ;) But soft mallets are fine, and not while playing, obviously.
For more effects, check out my teacher's book! There's tons!
Jerdol, that weird pedal sliding thing you mentioned is called, in fact, a pedal slide! I love them, personally. I know you've only been playing for about (well, now) 3 years, but have you seen caplet's l'espagnole? It's chock full of pedal slides, and awesome.
