You're right to be uneasy. Signing a contract without having a pro look at it is a scary thing to do. The Writers' Guild vets contracts for members, but I gather you're not a member, so that's no help. One thing you could do is try to get a literary agent on board. If you had some in mind, or if you have any subs out to agents, you could try emailing them with a header that will draw quick attention: "Have firm offer, need agent!" e.g. The best result is that you end up with both a publishing deal and representation. Even if they're not prepared to take you on, some might agree to look over the contract and advise you for a flat fee. Most agents know far more about the pitfalls and dangers than attorneys. You may not like what you hear, especially if they point out potential deal-breakers that the other side is not willing to negotiate; that will leave you with a tough decision to make. But at least you'll know what you're getting into.
If none of that works, make sure you have a good reversion clause that returns all the rights to you after a set period of time, or if the publisher goes under, or if it fails to publish your work within the agreed-upon period. That way, even if it goes bad, there's an end to it.
All this advice aside, take some time to celebrate! Someone liked your book enough to make an offer--that's a very big deal. Congrats.
