IAL D1 for maths

It just has the same cover style as the other new syllabus books. You didn't mention having trouble finding the other ones. so I dunno if that's just bad luck or they just sold out. If you're really desperate you can use the old D1 book. They only changed the travelling salesman chapter so now it has a bit from D2. (doubling weight of spanning tree to get upper bound, finding shortcuts to improve that upper bound, using the nearest neighbour algorithm to find upper bound, deleting a vertex then use prims to find spanning tree and adding the 2 least arcs bacc)
I think that covers most of the new stuff in D1. Also they got rid of the matchings chapter

Can you specify please cause I don't know anything about Decision, also I found the other IAL textbooks for Pure 1-4, but my school doens't provide D1 and idk where to find one online.

What chapters do I do and not do? Thanks

If you can find the old D1 book, you only skip the last matchings chapter. You also need Chapter 3 (travelling salesman) from D2.