One bridge skill that every player can work on by themselves is how to evaluate the strength of a hand. High card points (HCP) are a reasonable guide but even novices quickly learn that some honours are worth way more than their 4-3-2-1 count while others are worthless. Holdings like KJ or KQ, for example, are frequently worth nothing opposite partner’s void. Even aces are not worth as much as usual, because they don’t help to develop any other tricks in the suit.
There are surprisingly few books that help players improve their hand evaluation. There are two that I would recommend. The Complete Book on Hand Evaluation by Mike Lawrence is easy to read: he has an engaging, conversational style, and uses many example hands to make his points. Look out in particular for his discussion of the box and the shell, two ideas that will help any aspiring player.
If you are an intermediate (or better) player then the best book to read is The Secrets of Winning Bridge by Jeff Rubens. Only the first five chapters discuss hand evaluation but those 70-odd pages are worth their weight in gold. Rubens explains why Qxx Axxx xxx xxx is worth significantly more when partner opens 1 than Axx Qxxx xxx xxx. Once you understand the principle of “in-and-out” valuation you will realise how you can bid your hands in ways that will help partner make better decisions.
(Note: Rubens’s book was originally published in 1969 and was reissued in 2013. Lawrence’s book was published in 1983 and is one of the few books he hasn’t revised in recent years. Keep your eyes out for a new edition if you can’t find it in your bridge library.)