What to Look for in a Coloring Book
First off, whether or not you will like a book depends to a large extent on the subject matter. Some people like to color landscapes, while others prefer animals or abstract designs and so on. You may like having one of each or simply collect them as you find them online or in a bookstore. But there are some things you should watch out for when buying a book:
Some papers are absolutely amazing for coloring in with pencils but terrible with markers and vice versa. Some books seem to use nothing more than copy paper while others are almost like card stock. Not every book you like will have the best paper but it's one aspect to keep in mind when considering a purchase.
Intricacy of Design
Even within coloring books of the same category (say animals) you'll find variation in intricacy and level of detail. Do you want to color in tiny spaces with pens or prefer large areas for blending and shading with pencils? For instance, Johanna Basford is known for her intricate designs while other authors focus on wide spaces.
Singe or Double Sided
If you like coloring with markers or wet media, you might want to get book with one sided pages so you don't end up sacrificing half the designs in the book. In this case, you don't need extremely good paper quality to begin with (since there isn't anything behind). I use my Inktense pencils in Magical City but the paper will buckle and won't lie flat. You might not want that, though I don't mind.
Some books have hardcovers, others have removable flaps, some come in postcard books which you can send off to friends and family etc. See which ones you like or get a variety in all of them. Nowadays you can get lots of cool stuff to color like playing cards or even mobile phone cases! So if you don't like books or prefer something more crafty/useful, there's always something else available.
Some authors consistently put out good quality books. Other books may be cheaper and reflect the price. But it doesn't mean they're worthless. You may like the subject matter (cute cats!) or the book may feature your favorite TV show/movie/books etc (Game of Thrones or Harry Potter). By all means buy them, they will still provide you hours of fun. Just be aware that they won't offer the same paper quality as say Daydreams by Hanna Karlzon.
Of all the books I've bought, there has only been one that I felt unhappy with and it was due to the poor paper inside. I always read reviews and watch flip through videos on Youtube before purchasing and this has served me well so far. I've resisted buying books with poor paper even though I like the art inside because I know I'll regret it. But not everyone is affected by or cares much about paper quality. So by all means, buy whichever book catches your fancy. Just be sure to read reviews first and you won't be taken by surprise when you open the first page. Happy coloring everyone!