How can I get a flower effect to come down running on my custom made book stack? Also what is best to do this on Adobe illustrator?
If it happens that you want only an image, not actual printed books, you can place the same image on a few books as surface texture. The books are not equally sized, but the scale of the texture image is the same on every book.
If the texture image is placed differently on every book it's possible to get at least approximately contiguous looking texture pattern but it looks right only from one viewing direction. An example with 2 books (=solid extruded blocks with no fine structure):
The "books" stacked and watched from the right direction:
Unfortunately the possibilities of the used 3D CAD program stop here, there's no way to adjust lights nor let the books to make a shadow or a reflection. It's not a 3D modelling program, it's made for making mechanical drawings. I guess Blender or Maya users would laugh if they saw this. But The scene can be edited easily in Photoshop if wanted.
The whole job is possible in 2D, but warping and shading the image for plausible perspective and curved surface needs much work. I'll try to use a crutch. It's 3D effect Extrude & Bevel. It can be used to make the books if the extruded shape is simple.
Generally Illustrator's 3D effects splits item surfaces to difficult to predict pieces which makes artwork mapping complex, but extruding a four node shape which has no stroke makes as few as 4 surfaces and your perspective image needs only one of them to be textured.
Here a simple grey shape is extruded to resemble a book. The photo is dragged to the symbols collection to be used as surface texture. The photo looks small, but it has 1000x1000 pixels.
The light must be adjusted later to make it match with another book. The texture is placed in the next screenshot. It's stretched a little to make it look less distorted on the front cover side:
The book is duplicated, the copy is stretched to a smaller size and its extrude depth is reduced. Placing the copy and its texture carefully and readjusting the light settings in both versions the final view at least resembles continuous image texture on 2 separate books:
I must admit the 3D effect settings had to be reopened ten times before this result was got. Fortunately it's easy, only click the effect in the appearance panel and set Preview=ON.
It's still far from perfect. There's poor contrast and used plastic shading creates white gloss which can look wrong if the texture image tries to present something matte. In this case the gloss can be acceptable, but the books are still separate items, their lights and shadows do not interact. Illustrator makes one piece at a time.
Low contrast and too white glosses could be fixed in Illustrator by adding more light and by using diffuse shading. The missing interaction can be fixed by copying and pasting the books to separate layers in Photoshop:
The smaller book has got a shadow. Layer styles are not used, better control is got by inserting a blurred low opacity black copy which is moved a little downwards.
On the top there's a Curves layer to restore contrast. As said, contrast could be fixed also in Illustrator by adjusting lights and shading principle if the texture image itself has high contrast.
There's still no glossy reflections of the books on each other. I skip that problem.
As the term implies, this is a custom print setup, where each cover is prepared as a separate file, each using the same flower image background, but shifted each time, based on the thickness of each book, which needs to be calculated based on number of pages in each book, thickness of paper in each book, etc.
If you're just looking to simulate this, so not actually producing the books, but instead create this digital image of the stack, then you need to use Photoshop, apply the flower texture as an overlay covering the stack of books, set to multiply, and cut off the excess from the flower image, following the stack shape.