Let me start by saying I read the whole thing cover to cover. I do not recommend this method. The LCK can be read just fine in sections. You can start nearly anywhere and skip around as needed, just like the online one.
The entire thing is a little less than 300 pages. This was a very good length for me. Books that are much longer than that seem like such a daunting task I am intimidated into never picking them up to start. If the LCK had been much shorter too much would have had to be taken out. As it is Rosenfelder covers massive subjects, but manages to keep it compact to only things that conlangers need to know. Conveniently, there's an index at the end of the book so you can get more of the subjects, and learn the 'nice to know' stuff. He also manages to keep a dry subject interesting. Most of the example sentences are at least quirky if not down right funny.
The first few sections of the print version are nearly identical to the online one. These are sections like sounds, word building and grammar. They are expanded on just enough that I would recommend a beginner conlanger to read the print over the online version, but you still aren't going to miss too much if you just read the online one.
Where the print LCK really starts to shine though is the second-tier sections like syntax, semantics and language families. There's a lot of information here that's either not well embellished in the online version, or completely absent. This is really the stuff I was looking to read, and Rosenfelder delivered. He manages to take enormous subjects and include just the parts you need to know for conlanging. It also includes a very nice word list. Tools like this are invaluable for someone like me. Building up a lexicon is the most tedious part of conlanging, and the only part I don't directly enjoy.
I'm a little biased to the LCK, as I've written about before. I would recommend this book to anyone who is just getting into conlanging, or to an established conlanger looking for a reference book.