One of the negative aspects (and some may not find it negative) of reading on the Internet is the huge amount of information available on many topics. Researching a topic may be overwhelming and choosing the most important information may be difficult. While some people may enjoy having such vast resources, others may struggle to process and develop the material for its intended purpose.
When you read a book, you know who the author is and what his credentials are. This isn't always the case with material you read on the Internet. Much of what you read on the Internet is either opinion-based or not verified. If you plan to use material of this nature, you will need to do some backtracking to verify some of the facts. This might result in more work than you may have had to do if you'd gotten the information from a book focused on your topic.
Non-linear presentations can be an issue with reading on the Internet. Since you may end up reading many sources on the Internet, the material will not come in the logical presentation you might get when reading the same information in a book. Unless you find a comprehensive website that covers a topic fully or know the topic well yourself, this can make it difficult to digest the material.
Many people find reading a computer screen more difficult and tiring than reading pages in a book. This can result in a lack of attention that may cause you not to process what you read as well as if you were reading it from a book.
Reading on the Internet takes a little more discipline than reading from a book. When you open a book, you're in the world of that book. Outside elements can distract you, but the Internet offers everything from games to music to videos, making it easy to lose focus and find yourself off on random Internet searches when you get bored with what you're reading.