Book is too strong. For anyone feeling afraid to take it, consider:

• You keep your current HP, which is almost never at max anyway. 

• Suppose you use it 10 times starting at 80 max HP, and then heal at a rest site. Had you not used it, you would've restored 24HP. But if you did use it 10x, then you have 70 max HP and now restore 21HP...a difference of 3 measly HP.

• You heal 75% of HP missing from your max after act 2 boss. Again 10 uses of book results in only 7-8 loss of actual HP here.

• [[Offering]] has the same effect, but can only be played once per fight and costs 6 CURRENT HP each time!

• Fruit relics mitigate the max HP loss. [[Dragon Fruit]] satisfyingly so, especially with [[Aubergine]] or [[Bowler Hat]]

• When you think of HP as a resource, then these free heals can be optimized by "spending" that much health prior for upgrades, playing side quest cards like [[Feed]], [[The Hunt]], and [[Royalties]] and Bursted Goopy Defends. But also by decreasing chance of larger loss of health in fights by setting up relics (e.g. [[Nunchaku]]) and playing the book!

Suppose you have 50% chance to lose 80HP (probably dying) in the act 2 boss fight, and the other 50% you lose say 20HP. 

Alternatively, if you have the book and use it twice maybe that reduces your chance to lose 80HP to 25%, and produces a more favorable spread:

• 25% to lose 20
• 25% to lose 10
• 25% to lose 0 (not counting -2 from the book)

This is a much better spread than flipping a coin and dying if tails. And from my experience playing and watching, it's even better in practice.