The following are apps I use when making covers

PicsArt - This app is a must have. You can literally make anything with just this app. It's free on the App Store, but you can pay for stickers.

Superimpose - I got this app for $1.99. It's not a train smash if you don't have it, but I find it easier to blend using this app than in PicsArt.

Phonto - Adding text to your covers has never been easier. I'd definitely say this is a must have. The app allows you to download fonts and install them [there's a chapter on how to do that in the book].

Aviary - Not really a necessity, but I find it easier to Adjust pictures here. There's also really good stickers. If you sign up for an Adobe account, you can get stickers, filters, etc. for free!

Afterlight Legacy - This app is now free but there's an Afterlight 2 and idk how it is, but yeah. It's not necessary either, I just really like the textures on this app.

SplitPic Pro - I bought this app for $1.99, but there is a free version. It's a very simple way of blending pictures together - so really good for backgrounds.

Pixlr - This app has some pretty good filters and textures. I personally don't use it often, but it's good!

With all this, the ones you'll definitely need are:
PicsArt
Phonto