As a relative latecomer to this music I found the Ian Carr book invaluable.
I have Trevor Barre's first volume of the history of British free music ('Plink, Plonk, Scratch') which I really enjoyed. Sadly the second volume is out of print and goes for daft money when it comes up.
I made it about halfway through Ben Watson's Derek Bailey biog before giving up...
I'd love a copy of the compiled issues of 'Musics' but £95 is a stretch.
Are there any other good histories? Aside from the records themselves there's a real paucity of documentation.