Battletech Battlemech Manual
A self-contained hardcover, the BattleMech Manual is intended to be a mech-only up-to-date rulebook for tabletop use. It contains the majority of the most-used equipment and rules expansions from Total Warfare, Tactical Operations, Strategic Operations, and Interstellar Operations that are relevant to typical mech-only games. However, the most niche equipment and optional rules are not included, making those books still necessary if you're a completist. Construction rules are not included.
New material is limited to nine new Quirks, a complete lists of quirks for all mechs that had not yet had any assigned, a variety of simplified advanced terrain and fire rules, and a new quick-play system called Battlefield Support, which allows you to add artillery, air strikes, and minefields in an abstracted quick-play fashion.
While the rules in the Manual follow the standard BattleTech ruleset as presented in Total Warfare, in response to complaints from players they were rewritten and re-laid out from scratch in order to focus on clarity and ease of use at the tabletop. As such, the Manual is much easier to reference than Total Warfare.
With reference to the 2019 box sets, the BattleMech Manual is suggested as a follow-on to the "A Game of Armored Combat" box set