It seems everyone these days hates ISDN! WHY??
I like learning about it, it's not that hard. I always found WAN technologies easy to grasp even from when I started the CCNA.

Here's a nice
ISDN tutorial
There are some Docs on
ISDN zone about isdn products they offer and their take on it

I would also recommend the
Broadband Telecommunications Handbook for any aspiring networking god(It covers ISDN too). It is an amazing in depth book that I use as a supplement and it covers everything from satellite to mmds to ATM to whatever - I'll be using this one up to IE level :P
Thoughts?!?