Introduction to BattleTech is $18 ($30+shipping if you want a physical copy of BattleTech Beginner Box).

• CAT-35001 Total Warfare (2006) - all the BattleMech scale combat rules
• CAT-35010 BattleMech Manual (2017) - gathers all the Mech combat rules from various rulebooks
• CAT-35092 Tamar Rising (2022) - Clan Jade Falcon occupation zone from 3151 to mid-3152
• CAT-35136 Technical Readout: Clan Invasion (2019)
• CAT-35223 Era Report: 3145 (2013) - history and background material
• CAT-35400 Total Chaos (2012) - Compilation of Chaos Campaign tracks from various PDF releases
• CAT-35410 Battle of Tukayyid (2020) - The Clans vs ComStar. Chaos Campaign track with other material
• CAT-35621 Turning Points: Misery (2010) - Wolf's Dragons vs House Kurita. Turning Points are Chaos Campaign tracks.
• CAT-35680 Alpha Strike: Commander's Edition (2019) - Alpha Strike + AS Companion. Scaled down BTech for miniatures play with lots of units.
• CAT-35900 Shattered Fortress (2019) - splat-type book detailing 3146-3160 (end of MechWarrior: Dark Age)
• CAT-35901 ilClan (2021) - Battle of Terra
• CAT-35903 Empire Alone (2022) - Wolf Empire and Free Worlds League from 3151 to mid 3152
• CAT-35S003 Battle of Tukayyid Supplemental (2020) - 200 record sheets plus BattleForce material.
• CAT-35SN212 Touring the Stars: Tyrfing (2016) - Touring PDF series each are about 15 pages and describe a planet with some RPG and BTech stuff.
• CAT-35SN501 House Arano: The Aurigan Coalition (2019) - the setting of a computer strategy game.
• CAT-35TP016 Turning Points: Irian (2015) - Republic of the Sphere campaign
• CAT-35TP024 Turning Points: Tyrfing (2020) - Rumors of a Star League era cache bring forces from all five houses
• CAT-35TR101 Recognition Guide: ilClan v1 (2020) - First in a series of 24+ PDF supplements each detailing about 20 Mechs with record sheets. The best will be compiled into a future print volume.
• CAT-BTCCSW Chaos Campaign: Succession Wars (2019) - freebie campaign rules for linking BTech scenarios, for example adding supply and repair rules.
• BattleTech Legends (2021) - 52 personalities and their Mechs
• BattleTech: Primer - freebie PDF introduction to BTech
• Combat Manual: Kurita (2016) - Alpha Strike supplement
• Combat Manual: Mercenaries (2016) - Alpha Strike expansion
• Field Manual: 3145 (2013) - splat-type book with concrete game material
• Pseudotech: Arcade Operations (2022) - April Fools PDF with optional arcade-type rules for BattleTech
• Spotlight On: Crescent Hawks (2016) - small supplement detailing a mercenary unit featured in early BattleTech computer games
• Technical Readout: Succession Wars (2017) - a compilation volume containing Mechs from TR 3039, 3050, 3058, 3075 for use with BattleMech Manual.

So what we're getting is the main rules (Total War), Chaos Campaign for linking scenarios, some books that combined have lots of scenarios, a few splat or info books, a bit of Alpha Strike miniatures play. Mostly current timeline or so (end of Dark Age, start of the next era).

I guess not a bad place to start and $18 is a great price. BattleTech is such a large universe and a product line that's been in pretty much continuous production since first release. And with all the PDFs lots of compilation books and such. It's really all quite confusing and I'm not eager to jump back in so I think I'll be skipping this.