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Been looking around for a while but not really seen anything of use here.
Book 7 under the secutarii states that if a titan is in the same army, the secutarii can be mandatory options.

Past this, there is no classification or further info as to what a titan actually is.
While I know warhound and bigger are indeed titans, what about knights?
From book to book it changes between knight titan and imperial knight.

Anyone have any info on this please?
Searched the FAQ but found nothing.

Which 30k book(s) refers to Knight Titans?

I assumed the rule referred to Legio Titanicus units but it doesn't actually state that.

30K doesn't have the keywords the way 40K 9th edition has. Quite literally, you have to look at the name and title of the model. "Warhound Scout Titan" "Warlord Reaver Titan".