Yes, it is. Just make sure the flowers are dry enough before putting them inside the books :)
There is one thing to keep in mind though, different colours and textures of the flowers are the key here. I tend to pick up flowers with bright colours like dandelion or dark purple like the one you saw in my post (I made the birds).
Small flowers like daisies are great too. They are really pretty and last for long. I also love to dry Purple Hydrangeas (Cam Tu Cau Tim).
Flowers with thick petals are more difficult to dry with the "book" method so I haven't tried them.