A reader on Faeit 212 inquired to why Imperial Armour 13 was no longer available, and the reasons were were that some of the models in it, where discontinued.
|page found when searching forgeworld for IA13|
via a reader on Faeit 212
Imperial Armour 13: War Machines of the Lost and the Damned.
Apparently it was missing some models (which FW discontinued) and will not come back due to this.
IA1:2nd Edition went out of print as well due to the same reasons. I guess that IA3:2nd Ed. (Taros Campaign) will go as well, because that book features even more missing units like the Kroot and many Tau vehicles.