Sorry, I forgot to mention the for dummies book.
I've got a copy of the 6th edition of the all-in-one for dummies linux book too. I think it was part of the same bundle I got the LPIC study guides in.
From a quick look - it looks like a fairly complete reference book.
It mentions the LPI linux essentials course towards the end of the book - as well and an overview of the knowledge you need to pass the exam, but it doesn't go into detail, or tell the user to refer to any particular sections of the book. I haven't cross referenced this section to see if all of the things that are mentioned in that part of the book are actually included in the book.
The all in one for dummies definitely looks like it might have all you need in there for the LPIC1, but it may, or may not be missing information you might need to know. I don't know for sure!
But the LPIC guides are specifically aimed at people studying for their LPIC. So you are guaranteed to have all of the information you'll need to pass the exam. Which is why I recommended them.
So as reference books go, the all-in-one for dummies book looks like it should be pretty good. But as for whether it's good enough to learn what you need for LPIC - I couldn't say for sure. Which, again is why I recommended the LPIC study guides!