Learn English – Why communication skills and not communicative skills

A compendium is a concise collection of knowledge pertaining to a single topic or theme. Wikipedia states that the Bible is a compendium as it is a collection of apostolic writings (the New Testament) and similar (the Old Testament). A book containing all of Newton's findings (or most of them anyway) would be a compendium.

An encyclopaedia is more extensive than a compendium. It does not slot itself to a particular subject. It provides very brief information on almost everything that one should know about. No encyclopaedia can be exhaustive and one can always argue on how it decides what people need to know about and what not. Encyclopaedia is more of a reference material than a compendium; a go-to source if you want to summarily know about something.

An almanac is more about date-based and time-based information. It consists of dates of events, festivals and information on weather, agricultural and marine times and so on.

So if you would like to know when spring season arrives in Zurich, you would want to refer to an almanac. If you want to know what an aubergine is, you would refer to an encyclopaedia. Or, if you like to know the findings of various geographical discoverers, a compendium based on the same would be better suited to your purpose.