A quieter place to read.
A reading app for iOS — for people who annotate, look up new words, and return to what mattered.
Read the manifesto →
Casu is for people who keep a stack on the bedside table — who annotate the books they love, look up new words while they read, and return to a sentence the next morning.
Reader
EPUB, PDF, web articles, RSS, plain text. One calm library for every kind of reading.
Annotation
Highlight, write notes, sketch with Apple Pencil. Everything stays organized and exportable.
Vocabulary + Listen
Tap any word for a definition or translation. Captured words return on a spaced-repetition schedule. Listen Mode reads to you, paragraph by paragraph.
Sparks
A private journal for thoughts your reading sparks. Tagged to the book that prompted them.
Circles
Small private book clubs with shared schedules and a Bluetooth-proximity ritual that marks the moment you all gather.
The Invisible Handshake
When your book club gathers, Casu uses Bluetooth proximity to notice. As each member arrives, a quiet ring forms around the room. When everyone is finally close, the ring locks in. No sign-in. No ping. Just presence.
Read the full essay →Something happens to a book when you mark it. The page becomes a conversation — between you and the writer, between the reader you were and the reader you are now. I built Casu because I wanted somewhere to keep that conversation, quietly, for as long as it mattered.
I'm the person who built Casu. I read more than I should and worse than I could, and for years I tried every reading app I could find. Eventually I built one I'd actually open at night. If any of this sounds like you, I'd love to send you an invite.
— The Founder
Common questions
›Is Casu on Android?
No. Casu is iOS-only. We made the call so we could build deeply for one platform instead of compromising for two.
›How much does it cost?
Free to start, with a 3-document library and unlimited annotations. Casu Reader unlocks unlimited books, Listen Mode, and unlimited book clubs. $49 a year, or $6.99 a month.
›What's a Circle?
A small private book club. Set a reading schedule, pick a book, share thoughts in a private feed, and meet in person. When you all gather, the Invisible Handshake quietly recognizes the moment.
›Can I import my Kindle library?
Casu reads any DRM-free EPUB or PDF you import via the share sheet or Files app. Books locked into Amazon's ecosystem stay there.
›When will Casu launch?
We're inviting in small batches now and rolling out to the App Store soon. Request an invite above.
›Is my data private?
Yes. Reading data syncs through your iCloud account. Sparks (your private journal) never leaves your device unless you choose to share. We don't sell, share, or train on your reading.