Skippable books

[Adam]

It would have been fine to an extent. I can understand killing Tom, because there's a strange irony in there where he was the first person Jake wanted to save. Killing Rachel seemed to be simply for a massive shock value, to make the final book seem climactic, which is also understandable but less so.

As for Jara, that seemed completely pointless. That didn't need to happen, and it was unnecessary sadness without a pay-off or any sort of literary justice. Same with the Auxillaries, but at least they had a kind of death scene, and weren't just mentioned as a side note. Jara deserved better.

[Chad32]

The guy that proved he wasn't a controller by slicing his own skull opened deserved more than one throwaway line. The auxilaries deserved more than being cannon fodder. It would be kind of a dark irony that Tom's rescue was the first mission they went into, and making sure Tom didn't become the next Eva was part of their final mission. Rachel's death was just for shock value. She was the strongest of them all. If anyone was going to pull through, it would be Rachel. But they wanted to make her seem like a hopeless cause, dispite the end of book 48 where she sees through Crayak's faustian deal on her own.

[NickDaGriff]

Rachel wasn't a hopeless cause, but she was scared she might be.  Remember, a lot happened between #48 and #54, she just wasn't the character in focus.

[Chad32]

We usually see the best of her when she is the narrator. What she is on the inside is different from what she shows on the outside. I'm not saying she didn't skirt the line, but I do think she could have pulled through.

What was the worst thing she did in the series, anyway? If there was a moral event horizon for her, what would it be?

[NothingFromSomething]

I think generally speaking it was more the ghost writers that wrote Rachel as some bloodlusty head-case, the K.A.-authored ones are way more layered and three-dimensional.

[Chad32]

Some characterization did suffer from the ghostwriters not really understanding them the way their creaor did.

[Alan Fangor]

Well, with a long series of books, most of them self-contained, it's inevitabile that many seem "skippable".
Also because they are narrated by a different character, so every time it start from a different POV, a different situation that usually is not connected to the previous...the exception is David trilogy or, in a more vague way, 32 and 33.

Then I would be more cautious in defining them "fillers".

The first 8 books are necessary to explain all the main characters. Book #2 may seem unuseful but it shows other morphs, the life of a Controller, and some group dynamics. And Rachel's family, and, of course, first book narrated by her.

IMHO, the very "filler" or unuseful books are 24-39-42...in conclusion Helmacrons  . Other books are in fact useless for the plot, e.g. #11 The Forgotten, but it's quite interesting and introduces Sario rip.
Also #32 is avoidabile, but ignoring the absurd story of the two halves, this book talk about the Anti-Morphing Ray that will be important for the next book.