I've been using Wireshark in my job(s) for many years. Before Wireshark I used Sniffer Pro and before that Sniffer DOS luggables. Those I've been using since February 1994 when I was the Network Manager at Frye hospital in Hickory, NC. I started in Networking during the summer of 1988 at Eastern Data. While there I installed Netware 286 (eventually Advanced and NW 386) on ARCnet (eventually Ethernet and 56k and T1 WANs) for businesses all across North and South Carolina. I kept on doing that till I took the job at Frye.
In 1995 I went to Dallas, TX to work for Perot Systems where I finally got actual Network Analysis training (though I'd already learned much on my own). The training came from Pine Mountain Group (evidently now, Net Performance) who will teach you more about Network Analysis (ie: using packet captures/sniffers/Wireshark) than you can imagine, I highly recommend their training. For the last 11 years I've been at AOL where at this point in my career I don't get read network sniffs nearly as much. Though I still get to whip out my skills every once in a while for the new guys who think they know how to read a sniff because they installed Wireshark.
I highly recommend Interconnections, 2nd Edition by Radia Perlman for a good primer on networking. Radia writes wonderfully and you'll learn tons without falling asleep everytime you pick it up. And of course as everyone knows, TCP Illustrated, Vol 1 by W. Richard Stevens... though I can't say I recommend reading it straight through, wait till you want to learn something and then read everything about that subject. Understanding TCP MSS for example is an important subject for you in these days of tunnels, read all about it in TCP Illustrated, Vol 1.
Recently I posted on my blog about changing the font and icon size in Wireshark on MacOSX (and probably Linux too).