You mean the book that Taurian fans hold as they cry themselves to sleep every night? ;)
To be fair, just about all Periphery fans have a rather fond place for this book in their hearts, just like House fans love the HouseBook of their dreams. ;)
In all cases, those old books carry the most information about the Succession Wars and the periods before them. In word count alone, the amount of information given is just amazing. In later books, entire pages are condensed into paragraphs. Massive numbers of stories are simplified from sidebars a page long to, at best, a single sentence. Entire stories are simply edited out for page count so more detail can be given over to all the stuff that has happened SINCE 3025.
The sheer number of details given in those old books makes them pure bliss to someone who is looking for datapoints. Somewhere, I have a spreadsheet with planetary founding dates noted. I read through the books and noted when a planet was first shown in history, along with a page ref. The sheer number of worlds they mention in passing "so and so raided this world, so and so captured this world from whathisname, so and so colonized this world," and on an on is a note-taker's dream come true. :)
Also those old books gave the best looks into how things actually WORKED in the factions of the day, and no books even really tried to match them until the recent Handbooks. For people who love to ROLE play in the BattleTech universe, not just put some 'Mechs on the table and blow stuff up, the original House Books and the new Handbooks (which aren't as detailed as the House Books but at least try to fill in the same area) are the most important books ever printed in the BattleTech universe.