I have one adopted and one bio child, different order than yours though, we adopted first. I do not have a book to recommend, but I do have life experience. What my children have been told is that children come from different places. When my husband and I got married we had to find Allison, and the only place to find her was growing in the other lady's tummy. Once we had her, it was time to look for Ryan. The only place we could find him was growing in my tummy. Very simple and they love it. So you may tell your son that you had to find him in your tummy, but his new brother or sister may not be in your tummy, so you have to find the other lady whose tummy he/she is in.
Adoption can be very confusing for grown ups, so many choices and avenues, and explanations to friends and family. But to kids, the simpler you leave it, the better. A 5 year old's view of the world is different. Give him the minimum amount of information, then answer any questions he may have. Too much info may overload him. Focus on having the new baby, not where it came from, right now.