It's natural to try and combine rooms when you haven't got enough space, but many people use their home office as an ad hoc library to store their books. Granted, sometimes they end up in piles near a desk, but books are something that bibliophiles love, so they are worth making the effort to store them adequately.
In order to maximize your storage space, we recommend getting wall to wall bookcases or shelves. You will be able to find some that fit standard rooms, but anything higher than the norm will most probably require something custom-made. However, there are ways of using some simple shelving solutions to get the same result somewhat cheaper. Mix-and-matching shelves from the same manufacturer will often yield pleasing results.
These shelves, like the IKEA Billy series, will require the addition of some higher shelves so that no space is wasted. The wall-to-wall bookshelf look is pretty cool, especially when most of those shelves are filled up with books. If you find matching smaller shelves that can be stacked over these, then you'll end up with what you want.
Long Horizontal Shelves
Our preferred option is to use long horizontal shelves. We have the discontinued IKEA Enetri in our living room home office, and it works well to hold almost all of our books. The IKEA Stolmen is another option, but those can get expensive if you are going to deck out a whole room in them. The good thing about these is that you can add floating shelves above these shelves to complete them and get that wall-to-wall book look. You just need to match the color of the shelves, or hack them the right way, in order to get it done.
Once you've sorted out your shelving, it's time to find the right spot for your desk. Since this is a place to read books, you'll need to find a way to get a comfortable chair in there, with a bright light so that you can read for long periods of time. This will also be useful when you want to change positions from your main workstation. In our experience, it's best to place the desk facing some shelves, or off one, using something like the IKEA Expedit desk. A simple, low-key workstation desk will work best. The positioning of your desk will allow you to hide your cables, peripherals, etc, against the wall and even use the shelves right underneath your desk as miscellaneous storage.