As a huge fan of Clan & Valley, I have to agree with the other reviewers that Mammoth, Plains & Shelters fell short.
I have been hooked by the storyline of the first two books. Shelters (and Plains) is like a meal that looks delicious but has no taste, you will keep taking bites in hopes that the next one will have flavor!
Ayla has become, in her quest to fit in, normal and boring. Perhaps what the character wanted but not what the readers hoped for.
I found myself flipping through pages and pages of research references on the regions plant & animal life, cave dwellings and complete rehashes of the past 4 books over and over again.
I found that the Zelandonii had lost their primal ice age quality and had taken a Flintstones type of modern stone age way of living. Next we will find out how they had refrigerators and microwaves made of sticks and stone.
The book has lost touch with what I think most readers wanted to read about: Aylas uniqueness and struggles in an alien society.