Best Reference Docs for FI
 

As a prerequisite to an FI(A) course and use thereafter as an Instructor, what is/are the best reference docs in the ever changing world of EASA bureaucracy and legislation? Is there anything more specific than CAP 804? Thanks in advance.

As a FI CAP 804 is probably of little use to you, it will be more relevant to those you work for. If its legislation you are interested in then the relevant parts of the Aircrew Regulation EU directive 1178/2011, its amendments and AMC and GM are the definitive documents. I suspect you will really be more interested in what to teach than legislation, so why not try an EASA Instructor Manual

Standards Document 10 - Guidance for: Instructors, Instructor Trainers and Flight Instructor Examiners (FIE). (Aeroplanes) Assessment of Compentence for Instructor Certification
Also theres standards document 9 which is for the PPL examination. The course instructor will teach you the rest.

FI Docs
 

Also look at SERA, or better still the Consolidation from CAA website which includes the UK exemptions and permissions. There's so much basic stuff that people are unclear about.......quadrantal (no more!) low-flying less than 500' agl? VFR at night, SVFR?

I do use CAP 804 - it's stronger than LASORS ever was and its where the CAA point people, but it's still unwieldy and not the official reference.

There's also a short guide/summary on EASA website which is good for a quick reference doc for things like pre-entry requirements, experience etc.