Authoring Foreign Language Textbooks/Practice

Reading is not enough to learn a language. Readers should practice contents they've learnt. Lessons should assist them in doing so by supplying exercises (e.g. phrases to construct or modify) with answers. Note, however, that this is where Wikibooks starts to overlap with v:Wikiversity. You may want to track down the department that would benefit from your book, and help integrate it into a learning stream.

Exercises should be simple and allow plenty of repetition. User:Swift's old math teacher called it (with a grin on his face ;-)) the bulldozer method when he assigned them homework with around a hundred series to be tested for convergence. The same principle applies here. Repetitions are the only way to get an intuition for how series, sorry; language, works.

Give readers plenty of chance to passively understand and then practice (reading, listening, and speaking) example phrases before producing (writing or speaking) their own.

Here are some examples of exercises:

Multiple choice
These are extremely versatile, yet can be created with varying degrees of difficulty. They are excellent as a first step practicing new content as the various choices help jock learners' memories and help form mental maps of grammar and vocabulary.
Specific questions about the text
This may seem really easy, but that's not a problem. Having students verbalise and explain things on their own helps engrain concepts in their brains. You can only explain things you understand. Only advanced students can answer in the language they are learning.
Fill-in-the-blank exercises
Slightly more challenging than the multiple choice, it requires the reader to identify sentence structure and pick the appropriate word. This is particularly useful for interjections, particles, articles and genders, but nouns, adjectives, and prepositions need a trigger (see below).
Identify specific grammar in text
Give readers a block of text and have them highlight/circle particular grammatical entities and structures. This practices deeper comprehension both of the text and the item to look for.
Changing form
Have the learner change a sentence from one form to another (e.g. from one tense to another).
Reading and listening
Have readers answer simple questions to test their comprehension.
Dictation
Practice spelling.
Arranging words
Rearranging words into a sentence practices word order and grammar while providing the building blocks so learners won't have to come up with the sentences themselves.
Identify errors
Have readers identify mistakes in a text you provide. This can be used to immunize student against all kinds of common mistakes.
Translation exercises.
Tests vocabulary, idioms, grammar and also the student's ability to express themself in a native-like way.
Free writing exercises
own dialogues and stories, mock diary entries and newspaper articles, summaries, opinion pieces, analysis (roughly in order of difficulty).

While more work to create, visual triggers (such as images) are more useful than phrases. Describing something about an image, rather than translating an English phrase allows users to concentrate on learning the new language independently, rather than in terms of how it relates to English. This is particularly important for languages that differ radically in grammar.