Book 6 is called 'Retaliation'.
The main story will cover a shattered legion (Iron Hand/Raven Guard /Salamander) force taking the fight to the traitors, primarily being Iron Warriors.
Dark Furies are finished but not on show yet.
IW seige tyrants are about finished but not on show.
Book 6 will include early rules for dark mechanicum (rules not units) and will also include basic rules for mixed legion armies (shattered legions).
The main delay with prospero is FW wanting all the pretty models done beforehand so that they can be used in all the artwork and page backgrounds, plus having the models done help with rules writing for those units (what wargear etc).
We will be seeing Prospero releases a while before Inferno is released.
Expect to see Thousand Onns head/torso/shoulder pads before the end of the year and space wolves stuff early next year, with the basics like vehicle doors, contemptors, transfer sheets etc coming far earlier than the book
Book 6 will also include new rites for all legions. Two examples were:
- Word Bearers: Gal Vorbak as troops
- EC: Noise Marines as troops
Also basic rules for BA, DA and WS will be included - including a rite or two, special equipment and legiones astartes rule. Argel Tal will be in a later book - not book 6.