General:
* Absorbed
* Captivated
* Immersed
* Preoccupied
* Ravished
* Entranced
* Enthralled
* Spellbound
Formal:
* Engulfed
* Submerged
* Consumed
* Invested
Informal:
* Hooked
* Lost in
* Tuned in
* Sucked in
More specific:
* Focused: if the emphasis is on concentration
* Lost: if the emphasis is on being unaware of surroundings
* Delighted: if the emphasis is on enjoying the experience
The best synonym will depend on the specific context of the sentence. For example, "He was engrossed in the book" could be rephrased as "He was captivated by the book," "He was absorbed in the book," or "He was lost in the book," depending on the intended meaning.