I would like to think that his book is much easier than other traditional Grammar books, for he describes how language works and doesn't prescribe the usage of English language. You can even judge from the alphabetical order of the grammatical "rules" (why put rules in inverted comma? Rules are never fixed for descriptive grammarians!).
As for where you start from, of course, you'd be confused. He doesn't begin with tenses first etc or the way we are taught by any English teacher at school. He starts alphabetically, and my understanding of this approach is that his book is kinda reference and you can go to it to seek clarification at an instant. The best part of this book is that it gives examples with incorrect usage too. So any topic of English that interests you or that comes to the fore front of your mind to learn more about, just go through it from that book.

If you read a bit about Michael Swan and why he writes books for English Usage, you'd understand that as a learner of Applied Linguistics and English Language Teaching, he is more into just explaining theoretical side of Grammar and he even claims this over his website www(dot)mikeswan(dot)co(dot)uk that he is more into descriptive Grammar (describing the way English language works) etc.

Hope that helps!