Publishing contracts are notoriously specialist and complicated. It is often what is missing as much as what is in the contract that causes problems for authors. Do not hire a lawyer to negotiate a publishing contract, unless they have significant publishing experience.
ALLI provides a contract vetting service at three levels.
1) General advice and guidelines. Contract analysis and advice is offered, free of charge to members, by Orna Ross and Helen Sedwick, co-authors of ALLi's book How To Sell Publishing Rights.
3) Contract Negotiation on Your Behalf. We have various advisors who offer a more in-depth contract analysis / negotiation help, for a flat fee. Their expertise will generally add a lot more value to your terms and conditions. Contact Sarah at info@allianceindependentauthors.org as a first point of contact, explaining your situation and she will guide you to the right person.