what are the best qualities one should look for to learn the most from a chess book.

"The best book is the one you fall in love with" - I believe it comes from GM Kaidanov interview. I confirm it 100% percent.

Plenty of diagrams.  It is fine to just have reams of analysis if you have good visualisation skills but most of the reading I do (of any type) is on the train to work and I appreciate being able to see at a glance where a particular thought of the author's is coming from.

The author or authors must be able to explain their moves well and the resulting positions well.

Most books on openings are on only one part of one opening. For example the Rossolimo Sicilian by Bologan [excellent book]

If an author is trying to write a comprehensive book on a whole opening--that is very hard.  Example the Pirc in Black and White by Vigus--[another excellent book]

Also there should be current games as part of any opening book to help with the latest theory and it is a good idea to have games from the past just as a start.

Also, plenty of diagrams--2 or 3 a page and closer to 3 is maybe optimum.

Smudgy diagrams and weird notation:

best explanation I heard so far.

I know this shouldn't matter, but I have noticed that I get more from books where it feels like the author is talking to you the reader and has made a connection by including e.g. personal detail or humour - it doesn't have to be much. Then I find I care more about both what the author is trying to say and the pure chess content. For example, I am working my way through Cyrus Lakdawala's Caro-Kann: Move by Move at the moment and, because of his entertaining way with words, am finding it easy to absorb the info.

Entertaining prose, fun bullet points and lots of diagrams, algabrecic notation and an index of lines in the back.

Yes, Index of variations is really needed in the back of the book. But not just the Index alone--must have diagram or diagrams with each index variation.

Should also have a Contents page in the front.

I am  a chess opening book author and wish I had inserted a little more humor [humour] into my books.

For me i prefer a book that gives that explains the moves without going into too much detail. It is also nice if they give some kind of background to the game in question.My guess is that i have only completely gone through only 10 percent of the chess books which i have owned over the years. It is easier to buy a new (meaning new to me) chess book than to struggle through one i am having difficulty with.I am sure i would be a better player if i had actually studied throughly every book i ever owned but like most chess players i am fairly lazy when it comes to my chess studies.