Also known as the LCK, the Language Construction kit is a tutorial on making conlangs for people who have little or no knowledge of linguistics. It was created by Zompist (Mark Rosenfelder) as a beginner's guide to the art of conlanging, describing the components of a language and typical ways that real languages work. It is available here.
In response to calls for an expanded LCK, Zompist wrote a book version of the kit which expands many sections and adds others. The book gives particular interest to often-neglected areas of conlanging such as semantics and pragmatics. It also includes a grammar of Kebreni, one of the conlangs created by Zompist.