There's a simple rule for when you need a legal review of your book contract.
"If the other side in the negotiations has a lawyer, you need a lawyer."
Authors Guild members can get a free Book Contract Evaluation from the legal staff of the Author's Guild. The evaluations take about 10 days, and they are concise, accurate and easy to understand.
If you are not a member of the Author's Guild, you might consider joining.
If you hire a lawyer, make sure he or she has extensive experience in book contracts.