Where to go from here
We have only scratched the surface of the TADS 3 library (and the TADS 3 language), but hopefully enough has been covered here to get you started. The next step is for you to experiment on your own; you could either expand the Further Adventures of Heidi with further features of your own devising, or else start your own (and probably rather more interesting) test project. I shan't make any concrete suggestions what else to add to the Heidi game, since it'll be much more interesting for to implement your own ideas; but I'll pose a few questions your extension of the game might like to answer: whose cottage is it that Heidi starts by? Why were the boots buried in the cave? Could Sally the shopkeeper be the object of Joe's affections? Is the stream at the bottom of the garden the same as the stream Heidi has to cross to reach the meadow, and does that suggest any other uses for the boat? If Heidi could walk into the village from the jetty, what might she find there? Where else could she reach from the meadow? Why was the key lying there?
As an alternative, you might like to turn to the Technical Manual and read the article on Designing Your Game. This discusses the design of a game based at an airport, and shows how to implement the first few steps; as an exercise you could try implementing the rest of the game.
If you start following your own inventiveness, either by expanding the Heidi game or by writing your own, sooner or later (probably the former) you'll come up against something that hasn't been covered in this Getting Started Guide, or else come up against something that you can manage by some convoluted means but which makes you think, "surely TADS 3 provides a better way of going about it than this?" Given the richness both of the language and the library it's quite possible that it does, the problem is to discover where and how.
There are several other places you can look. One is the TADS 3 Tour Guide which, like the current Getting Started guide, takes you through the development of a complete game. The Tour Guide, however, is considerably longer, covers much more of the library, and tries to work through all the main classes in the library in a reasonably systematic fashion. It should prove an excellent resource for extending and deepening your knowledge of TADS 3 (but then I would say, since I wrote it!). The TADS 3 Tour Guide should have come with the documentation included with the TADS Author's Kit, but it can be found on line at (and various formats downloaded from) http://users.ox.ac.uk/~manc0049/TADSGuide/intro.htm. It is also available from the IF-Archive.
Another place to look is is the TADS 3 Technical Manual, which again should be included with the documentation that comes with the TADS 3 author's kit, or should otherwise be downloadable from http://www.tads.org. This contains a variety of sections, some of which cover in greater detail material introduced in this guide, and some of which which take you into new areas (such as writing a TADS 3 game in the past tense). A third resource is the TADS 3 System Manual, also included with the standard documentation (or downloadable from http://www.tads.org). This may not be something you want to read from cover to cover in one sitting, but it is something you will want to refer to again and again, and you will need to familiarize yourself with much of its contents sooner or later. The thing to do first time round is probably to try to read through all the bits that look interesting and informative, and to skip anything that seems boring and obscure unless and until you really need it.
Yet another resource is the 'TADS 3 Version History' (the link near the bottom of the HTML page when you select the Help option from Workbench), and in particular the Recent Library Changes list, which can be a mine of information on TADS 3 features.
How you tackle all this documentation is up to you, but here's one suggestion for an order that you might find helpful. First go to the Technical Manual and read the articles 'Tips on Designing Your Game', 'Object-Oriented Programming Overview', 'How to Create Verbs', 'Action Results', 'Message Substitution Parameters', 'NPC Travel' and 'Creating Dynamic Characters'. Next turn to the System Manual and read everything in Parts I-III up to and including 'Anonymous Functions'. Then read Part IV, but skip over 'Input Scripts', 'ByteArray', 'Character Set', 'File', 'Grammar Prod', 'Intrinsic Class' and 'StringComparator' for now. Then read the section on 'Program Initialization' from Part V After reading 'tads-io Function Set' read the article on 'Some Common Input/Output Issues' from the Technical Manual. After all that you may want to have a go at the TADS 3 Tour Guide, and you should be well equipped to benefit from it.
But no set of documentation is going to tell you everything you will eventually need to know about the TADS 3 library. Sooner or later there will be no subsitute for looking at the library code to see how various classes and functions are implemented. Fortunately, the library code is well commented to guide you through understanding how things work. Even more fortunately, the documentation that comes with the TADS 3 author's kit comes with a set of linked HTML files (The TADS 3 Library Reference Manual) that make finding your way round the library source files much easier than it otherwise would be. You will find this an invaluable reference resource whenever you're programming in TADS 3.