If you are looking to improve your colouring techniques, then Helen Elliston has produced a series of books that can help you. There are four books in the series and they all cover different things. I find that there are always some things in them which I can apply to Johanna Basford books, but I am sure they will also apply to other authors colouring books as well.
The four books cover different items and they all have practice pages in them as well, so you can have a go at the techniques before trying them in your colouring books. The paper is not that thick, so I chose to practice straight into my colouring books. Each tutorial has simple steps and diagrams to help you.

The first book starts with simple things such as shading and blending. Then there are sections on different topics: hair, lips & skin tone, beautiful and delicate natures, 3D shapes & objects, gems and backgrounds. I particularly found the mushroom, gems, water droplets and crown useful. I adapted the crown tutorial for the gold colour in the picture to the left.

Colorist’s Special Effects 2 has sections on tasty treats, the natural world, 3D objects, metal & steampunk and skin tone. I particularly found the cupcakes, doughnuts, feathers, flames, flowers, frog, grass, ladybug, silver leaf, gems, shells, lighthouse, red heart gemstone, unicorn horn, star and metal spoon useful. The pumpkin and grapes in the picture on the left were coloured using tutorials from the book.

Colorist’s Special Effects 3 has sections on flowers, glorious nature and a section including fur, metallics and backgrounds. I particularly found the tulip and bleeding heart (as left), 3D filagree, fairy door, grassy meadow background, topiary, bee (I have a video tutorial based on this here), waterfall, tree bark, sunflower, Fuschia, pansy, cherry blossom, bluebell, cactus and orchid useful.

Colorist’s Special Effects 4 has sections on edible ingredients, bold backgrounds, beauty of nature, cool stuff (including ice cube, rosette and stained glass) and mystical (including potion bottles and feathers. I found the candy cane, gingerbread man, pink flower, squirrel, acorn (see left and click here for video tutorial) and potion bottles most useful.
I have also completed video reviews of each of the books and you can see the below:
If you are interested in seeing some reviews of the books and/or buying them, you can do so here:
Colourist’s Special Effects – https://amzn.to/33WxIPD
Colourist’s Special Effects 2 – https://amzn.to/346selz
Colourist’s Special Effects 3 – https://amzn.to/2Y412jv
Colourist’s Special Effects 4 – https://amzn.to/3kECNSu