These are both part of Apple's Everyone Can Code initiative, and both are appropriate for a high-school and college audience.
The Intro book is intended for non-programmers, and teaches programming fundamentals and Swift syntax, with 90 hours of lessons included.
The non-intro book gets into more complex UI development such as working with table views and navigation, and topics like consuming web APIs, and includes 180 hours of lessons:
App Development with Swift Curriculum Guide (where I took that screenshot) also shares helpful outlines of the two curricula that can give you a sense of the differences at a glance.
Here is the Overview from that guide explaining the difference:
The Intro to App Development with Swift and App Development with Swift
curricula were designed to teach high school and college students with
little or no programming experience how to be app developers, capable
of bringing their own ideas to life.
The Intro to App Development with Swift course introduces students to
the world of app development and the basics of Swift and Xcode. The
course culminates in a final project where they can choose one of two
basic iOS apps to build.
App Development with Swift takes students further, whether they’re new
to coding or want to expand their skills. If they’re already familiar
with Swift, Xcode, and iOS development, they can move through lessons
quickly or go straight to the labs, where they’ll build miniprojects
and test their code in playgrounds. By the end of the course, they’ll
be able to build a fully functioning app of their own design.
The document goes on to further explain the two courses as follows:
Intro to App Development with Swift
This introductory one-semester course is designed to help students
build a solid foundation in programming fundamentals using Swift as
the language. Students get practical experience with the tools,
techniques, and concepts needed to build a basic iOS app.
App journal activities take students through the app design process,
from thinking about the purpose of an app to market research and early
user testing. By the end of the course, students will have created a
plan for an app they’d like to develop. Even though they might not yet
have the skills to build the app, the work they put into the framework
will set them up for future development.
App Development with Swift
This two-semester course features 45 lessons, each designed to teach a
specific skill related to either Swift or app development. Each type
of lesson takes a different approach:
- Swift lessons. These lessons focus on specific concepts. The labs for each are presented in playgrounds so that students can
experiment with code and see the results immediately. Playground files
- App development lessons. Focusing on building specific features for iOS apps, these lessons typically take students step by step
through a miniproject. The labs help students apply what they learned
to a new scenario.