Fundamental Chess Endings (Muller and Lamprecht)
Fundamental Chess Openings (Sterren).
A good games database - The best free one is this http://caissabase.co.uk/ (It's my project so I might be biased but it is the best free one I've found, it exists because I couldn't find a great free one that was always updated at least monthly) and Chess DB software (ScidVsPC is open source).
That and playing and analyzing your own games is all you'd need (apart from a massive commitment to improvement) to get to Category A (1800-2000USCF.
If you feel like throwing in another (cheap) book I like https://www.amazon.co.uk/Mammoth-Book-Chess-Books/dp/1845299... as it's a weighty dense cheap book you can throw in a bag and not care about getting beat up.
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