Grave Covenant, by Michael A. Stackpole, is the second book in the Twilight of the Clans series.
The novel was made available on BattleCorps on 26 July 2010 as a PDF file (text only, without cover, pictures, or any other interior artwork except for the usual BattleCorps frame graphics). The PDF copy includes a disclaimer stating that it was created from a pre-final edition text that might differ from the printed version and that canon-wise, the print edition trumps the PDF edition.
From the back cover
||Possessing precious secret information that will lead them down the Exodus Road, the Inner Sphere Successor States can now meet the Clans on their own terms... and behind enemy lines. The Herculean task of amassing enough power means re-establishing the legendary Star League—a union of Successor States and their BattleMechs led by Victor Steiner-Davion. But with the deadly game of politics make the Successor Lords wary of alliance, and Victor's Machiavellian sister, Katrina Steiner, hatching more vicious plots to further her own agendas, the war against the Clans may only be a distraction from the real danger...
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Original 1997 cover. Art by Roger Loveless.