Shapechange is a 9th level spell that lets you assume the form of a different creature, replacing your game statistics, including your Strength, Dexterity and Constitution scores, by that of your new form (but keeping your initial Intelligence, Wisdom and Charisma scores, as well as any benefits from your race, class and other sources).
The Manuals of Gainful Exercice, Quickness of Action and Bodily Health are very rare magic items that can be used to respectively increase your Strength, Dexterity and Constitution scores.
What's ambiguous for me is the above other sources, when pertaining to persistent effects from magic items such as the Manuals. For instance, if a Human Druid, who has pre-emptively read a Manual of Gainful Exercice, casts Shapechange to turn into a Frost Giant Everlasting One (Strength score of 25), is their Strength score 25, or 27 because of the book's persistent magic ? Do you keep Ability Score Increases from Manuals when Shapechanged ?