The tabletop or boardgame of Battletech has long been the primary focus of the Battletech designers. In all previous incarnations of the Battletech RPG (with various names - A Time of War is just the latest), the Core RPG rulebook referred players to the tabletop game for mech stats, or to play games focused on mech combat. As far as I can tell, this edition is no different. Contrast this with a game like Heavy Gear that features a similar setup, but has vehicle/mech stats as an intrinsic part of the RPG game system.
All mech designs for Battletech these days are to be found in the TROs (Technical Read Outs). The occasional special design might be found in other books. However, Total Warfare, the current core boardgame rulebook, does not have rules for mech construction, meaning there is not even the usual example design in the main rulebook, unlike previous editions. Instead, the construction/design rules for all types of Battletech vehicles are all encapsulated in the the Tech Manual.
If it helps, there is an independant Battletech Wiki with descriptive details of almost all variants of mechs (but no actual stats). There is also an official Catalyst-maintained site that lists all known Battletech units, and their Battle Values, but basically no other design information.