Bell's book was one of the first I bought years ago. Although I enjoyed the book and learned from things from it, I agree that it is basically garbage. The reason I say that is the author created his own rules for cube evaluation, and is doubtful they hold up to computer scrutiny.
For example, Blitz Positions: calculate the blitz count
Blitz Count = (Attacker's Home board points) + (Defender's men on bar) - (Defender's blocking points) - (Attackers Runners)
Double if count 0 or more
Take if count 1 or less
Drop if count 2 or more
Too Good if count 3 or more
Interesting stuff but I don't believe it. probably food for thought but ignore it.
On the other hand, I found Bell's coverage of principles such as doubling window and market losers, value of cube, etc. access to be very good. Or at least i did when I was starting out way back when.
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I own the Cube action books as well, and to be honest, they mostly have sat on my shelf for years. There are no comments as Robertie already mentioned. I think the main value would be to treat it as a quiz book or flash cards. Look at the position, take a guess, and check the answer. You could learn something that way, but I never really did this myself.
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I own the new Woolsey vol 2 cube book as well. I was disappointed in this book as well. Woolsey/Robertie did not include the full computer evaluation. Meaning the position shows for example
No Double: .85
Double / take: .95
Douple /Pass: 1.00
but does not show the win percentages and gammon values. This is a big oversight which degrades the value of the book.
So what do I like?
Olsen in Backgammon from Basics to Badass has a 50 page section on the cube which I think is quite good. He looks at position types, and details the factors you should consider for both sides in that type of position. Doesnt necessarily give you an answer, but guides you to approach the position in the right way.
Stick Rice offers for sale a collection of annotated problems as XG computer files. He has a separate collection for blitz cubes and another for holding games. I found his brief comments enlightening. Several years ago I organized these positions into a book which I printed out. With Stick's permission, I offer a PDF copy to anyone who also purchases Stick's product.
Kit Woolsey's Backgammon: master versus amateur Vol 1 is a deeply annotated match between himself and his readers. While it isnt a specific treatise on the cube, he cover's Woolsey's law in great detail over and over again as positions come up during the match. I thought it was great.
Any book by Robertie is gold. In particular I like 501 Essential Problems and Advanced backgammon Vol1 and Vol2. While he doesnt specifically address the cube, he has many well chosen problems organized by theme which are a great learning tool.
If you can afford it, the Backgammon Learning Center provides one on one lessons and the course covers how to handle the checkers and the cube for each of 8 major position types. I endorse it, both as a former student and former teacher there.
You might also try
https://www.backgammonstudio.com//
This website has many quizzes, some of which are cube quizzes which could be useful.
Also try voiceofbackgammnon lectures on youtube. If you can find one with commentary, I find these particularly interesting to compare your thoughts with the host as the game progresses in live time. here is one I did earlier this year, but if you look hard you can find better commentators than I
https://www.youtube.com/watch?v=2JmSb_fQWpE
