Below is a list of some of the useful (& mostly free) tools I use for self-publishing graded readers:
1. Grading text
Online Graded Text Editor (OGTE)
Level conversion chart (comparing grading systems)
2. Reader photos and images
Adobe Stock (stock photos & images-not free but ‘free trial’ available)
3. Audio files
VLC media player (converting audio files to mp3 etc.)
Audacity (editing audio files)
4. Publishing
Kindle Create (needed to create eBook files for Amazon KDP)
Japan ISBN Agency (for books published in Japan)
Some of my other ER-related posts
What is extensive reading? How can it help me? (for students)
Ten tips to get students reading more outside class (for teachers)