Here is the path of stuff in a general way:
Intro Box
Total Warfare
Technical Readouts
Solaris Skunk Werks - Free 'Mech creation software
http://www.solarisskunkwerks.com
TechManual
Tactical Operations (We have a rule for that)
Strategic Operations
Now at any point after you get Total Warfare, there is a metric buttload of sourcebooks and other non-essential books available to you. On BattleCorps
http://www.battlecorps.com you can download a bunch of PDFs of old scenario books, sourcebooks through the decades Battletech has been around, and tons of other things like Record Sheets. Hint: If you get Skunk Werks, you only need Record Sheets for vehicles and infantry. Blank versions are available at the site Orlanth mentioned.
Also, head over to
http://www.catalystdemos.com and sign up as a player. You can find local Demo Agents (like me) and other local players.