Download zip file
https://artifacts.picoctf.net/c/502/files.zip
$ tree .
.
├── 13771.txt.utf-8
├── 14789.txt.utf-8
├── acceptable_books
│  ├── 17879.txt.utf-8
│  ├── 17880.txt.utf-8
│  └── more_books
│  └── 40723.txt.utf-8
├── adequate_books
│  ├── 44578.txt.utf-8
│  ├── 46804-0.txt
│  └── more_books
│  ├── 1023.txt.utf-8
│  └── .secret
│  └── deeper_secrets
│  └── deepest_secrets
│  └── uber-secret.txt
└── satisfactory_books
├── 16021.txt.utf-8
├── 23765.txt.utf-8
└── more_books
└── 37121.txt.utf-8
10 directories, 12 files
$ grep -r picoCTF{
adequate_books/more_books/.secret/deeper_secrets/deepest_secrets/uber-secret.txt:picoCTF{f1nd_15_f457_ab443fd1}