Hi, all. New here.
I just recently found this site and started rereading Animorphs, which I had quit reading at #44 many years ago (just when it was about to get good again, ironically). While I've been reading, I kept a record of which books seemed relevant to the overall plot.
Take this with a grain of salt wolfev, but here's what I've come up with. A lot of the filler books have short references in later ones or morphs than end up being used again, (Cassie's skunk, for instance) but these are what I would suggest to a minimalist.
Must-reads:
#1 The Invasion
#2 The Visitor
#3 The Encounter (Potential skip, but it really helps develop Tobias)
#4 The Message
#5 The Predator
#6 The Capture
#7 The Stranger
#8 The Alien
#10 The Android
#13 The Change
The Andalite Chronicles
#15 The Escape
#17 The Underground (You could skip this one, but it introduces lots of things about how the Yeerk Pool is defended)
#18 The Decision
#19 The Departure
#20 The Discovery
#21 The Threat
#22 The Solution
The Hork-Bajir Chronicles
#23 The Pretender
#26 The Attack
#27 The Exposed (Possible skip, but it introduces the Drode, who reappears a lot)
#29 The Sickness
Megamorphs #3: Elfangor's Secret (Possible skip, but it helps develop Ax somewhat)
#30 The Reunion
#31 The Conspiracy (Possible skip, but almost vital to developing Jake & Tom)
#33 The Illusion
#34 The Prophecy(Possible skip, but this one is fun if you read HBC)
#35 The Proposal
Visser
#38 The Arrival
#40 The Other (Possible skip, but it introduces Ax's bias against vecols)
#43 The Test
#45 The Revelation
#46 The Deception
#47 The Resistance
#48 The Return
#49 The Diversion
#50 The Ultimate
#51 The Absolute
#52 The Sacrifice
#53 The Answer
#54 The Beginning
43 books total, could be taken down to 36 for a hardcore minimalist.
Narrator stats:
Jake 7 (or 6)
Rachel 6 (or 4)
Tobias 6 (or 5)
Cassie 5 (or 4)
Marco 9 (or 8; KA must have seriously loved Marco)
Ax 5
Others 3
All 2 (or 1)
Other books that I liked a lot/might suggest reading for one reason or another:
#25 The Extreme (This one was just cool, plus they morph the Polar Bears again later)
Megamorphs 4 (Almost completely pointless, but interesting)
#37 The Weakness (I hate this one, but it is sort of a Rachel development book, I guess)
The Ellimist Chronicles (Again, I detest this one, but technically it could be considered relevant)
#11 The Forgotten and Megamorphs #2 (These are fairly cool, but entirely pointless)
Again, these are just my observations. I'm sure some of the books I haven't listed are worth reading to others.