Hey! So, a lot of people ask other book cover making people what apps they use. I think it'd be useful to tell you guys what I use. ^.^ I have an iPad Mini 3, so sorry if you only have a computer or Android device! Anyways, the apps I use are for iPad mostly, not iPhone. I have heard some computer recommendations so I'll post that too.
These are mostly free!

For iPad.

Text Apps
The best text app is Phonto. It has a lot of fonts, you can add new fonts, you can curve text and fade it, adjust the space between letters, words, and lines. You can even add symbols, which is good for watermarking. There are some secondary text apps, such as Write Behind, which does exactly what it says it does- it lets you put text behind your photos. Font Killer is good for 3D text, like tilting the text so it looks like it's on a wall or on the floor. You can also fisheye the text.
Photo Effects
Repix is good for really cool little things, like adding light flares or raindrops, making a photo look like cloth, things like that. There are really cool IAPs, like live brushes- where you can make it look like dandelion fluff is around you, like you're trapped in a storm of ravens, awesome things like that. Aviary is awesome for things in general. (Lol! So specific!) It has everything you really need, overlays, filters, textures, text, stickers, and even a meme maker! That is the one app you should download! Polarr is featured on the App Store as of June 2015. It's pretty good, but a bit confusing. There are a LOT of controls, and if you're used to super sophisticated photo editing like Photoshop, that'll be good for you. Photo Candy is ok, but Aviary is better. ^.^
Double Exposure effects
Well, I haven't actually used many apps like this. Some that were recommended to me are Fused, Edit.Lab (iPhone) and Gradient Blender (iPhone). Also, this was on AppsGoneFree- it's not free anymore- but Diana Photo is cool too.
Collages
PicCollage is my go-to app for collages. It's just awesome. Like, you won't know until you try it out. ^.^ It's the best collage app out there. I only have one secondary app: Fuzel Collages. It's OK. It has credits so you don't have to pay for extra stickers, so I guess it's good. It doesn't have many fonts though. It has a lot of stickers and labels. Also, I haven't tried this out, but there's Layout by Instagram.
Cool & Fun Stuff (Doesn't help with covers!)
A funny fun stuff one is Animal Face. Basically all you do is put animal faces on people! Plus, you can erase the face- say there's a hand in front of it- so it's more real looking. Plus, there are really cute borders. There's also a lot of cloning/levitating photo apps, and they're awesome to play with.
Other Awesome Apps
Procreate is the best art app ever. Like, seriously. There's no words for it. It has everything you could dream of. It's good for artistic touches on photos and just plain art. Seriously, Google Procreate app and look at the images. WOW. If you have the new update for MCPE- skins, finally! 💕 Get Skinseed Free. It's awesome for skins, make your own, search some up, (there's a lot ^.^) and even steal from PC skins. (Bacca skin, here I come!)
Yeah, I have a lot of apps. :)

Computer
Well, I don't photo edit on the computer. Obviously Photoshop is awesome, and some people recommend Pixlr. [ I DO ] I don't know if Aviary is still up. That's all, sorry! Canva is good for quick, simple book covers if you want to make your own. You don't need artistic skill, but the covers end up a bit plain.

Ok, that was much longer than I expected! Hope this helps!

BEST APPS: TL;DR: Phonto & Pixlr & Superimpose & Pixelmator
Ask for fonts that I like if you want.

~Esha