by Garfink
Get Classic Battletech: Tech Readout: 3039, the older versions of the tech readouts are the same as the new version and is totally compatible, so whichever is fine.You can also get the respective record sheet also.
You may want to buy Classic Battletech: Historical: War of 3039 if you're interested in more fluff about the era.
The 3057 tech readout has a revised edition from Fanpro. But the old one is almost the same.
You may be interested in this link, it has all the books available in chronological order of release. Useful if you want to know what is available.
http://www.sarna.net/wiki/List_of_BattleTech_products