The Art of War is truly a classic of general applicability.
Counter-intuitively in the context of Ukr coflict, another, more recent, cleassic has much to commend it:
Small Wars: their principles and practice by Callwell. Although written in the context of western imperial expansion and fuzzy-wuzzy bashing gungho days, we find that scale of conflict does little to confound the generalities.
I would add that Callwell's book, readily available, is a wargamer's bible, packed with examples written when partipants were still alive.