Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Margins: Book Tracker
Margins is a beautifully designed app for people who love books.
Free · In-app purchases
PRICE
Free
In-app purchases
BUSINESS MODEL
Freemium
No overall Grossing estimate available
RATING
★ 4.9
19,034 ratings · Top 5%
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
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Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What's included18 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
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Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Margins: Book Tracker: map the core Books workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Introduce a more visual and interactive progress tracker.
Assumption to check: Difficulty in tracking reading goals and progress across multiple books.
Validate with people who use this workflow before building.
Hypothesis 2
Allow users to create custom categories and tags for better organization.
Assumption to check: Limited customization for book categories and tags.
Validate with people who use this workflow before building.
Hypothesis 3
Implement a clearer subscription model with detailed benefits outlined.
Assumption to check: In-app purchases can be unclear or hard to navigate.
Validate with people who use this workflow before building.
Hypothesis 4
Add social features for users to connect and share their reading experiences.
Assumption to check: Lack of community features for sharing reviews and recommendations.
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
Margins is a beautifully designed app for people who love books. - App of the Day in 20+ countries - 4.9 rating (40,000+ reviews) - Featured in USA Today, BookRiot, and more TRACK YOUR READING - Log books, series, authors, recommendations, and more - Track Strava-style goals, stats, and streaks - Curate lists of books you care about SEARCH BOOKS BY VIBES - Search books by mood, plot, setting, vibe, and more - Try things like “books like 1920s Paris,” “thriller with a love triangle,” or “indie romantasy” - You can even turn images from Pinterest or your camera roll into book recommendations BLOCK DISTRACTING APPS - Block social media, notifications, and other apps while you read - Use meditation reading timer for soothing rain sounds CHOOSE YOUR COVERS - Pick the cover that matches your version of the book - Curate covers to fit your aesthetic - Sort your library by color SEE WHAT FRIENDS ARE READING - Find and follow other book lovers - See what they’re reading in Stories - Enjoy gorgeous, hand-illustrated monthly recaps CUSTOMIZE YOUR PROFILE - Turn your picture into a sketched portrait you’d see in a book - Show off your favorite books LEAVE GOODREADS FOR GOOD - Import from Goodreads, StoryGraph, and Fable - Half-star ratings - Support a small, indie team WHAT PEOPLE ARE SAYING “LOVE this app! so easy to import data from goodreads. it excels where others fail. like letterboxd for books. Really glad there’s finally a good book app out there, i’ve been waiting for something like this!” — scruffythejanitor01 “I love the UI of this app, easy to use and aesthetically pleasing. Also, I've been struggling getting back into reading and the screentime feature has helped me loads. I haven't been able to read this much since 4 years ago. Thank you Margins” — mitakaluvr “The search by vibes is so cool. I am a WWII buff, but this search popped up a two-book WWII saga I had never even heard of, much less read. It ended up being my top read of 2025.” —Michigancolt FREE TO USE Almost all features are free to use. An optional subscription unlocks premium features and priority support. FOLLOW US Instagram: @readwithmargins TikTok: @margins TERMS OF USE Terms of Use (EULA): https://www.apple.com/legal/internet-services/itunes/dev/stdeula/ Margins Terms of Service: https://margins.app/terms Privacy Policy: https://margins.app/privacy “Without the margins, writing is just words on a page. But with them, each of us is invited to leave our mark in the endless conversation between books and the people that love them.” — Excerpt from the Margins manifesto
Written user reviews are not shown here. Check current App Store reviews >
Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
React Navigation
Zustand
Expo
React Native Paper
Victory Native
Axios
BUILD KIT DIFFICULTY
Intermediate
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
