I'm so sorry to hear about your breakup. I think it is key not to blame the other woman - remember that a 3rd person cannot break up a solid relationship. Obviously there were flaws in your relationship - it sounds like your boyfriend has the issues, but if he didn't, she couldn't have broken you up. I think it is essential that the 7 year old know that Daddy loves him/her, and that Daddy is not leaving him/her. Explain that parents do not leave children. The child will be most fearful now that YOU will leave too. Explain that parent/child love is FOREVER, and that some families live together and some live separately. This is a good time to introduce the idea that there are all kinds of families - depending on your comfort level, you can perhaps use examples of families you already know: single parent, same-sex parents, grandparents raising grandkids, foster parents, adopted kids (particularly interracial families), etc. There's no limit to the amount of love a person can give. While it's too soon to tell if this woman will be in the picture (rebound relationships often don't work out), it will be important for your child to know that, if Daddy should be in love with this other woman (or anyone else down the line), that's more love for your children.
I'm not sure what's out there regarding books, but you can consult your children's librarian - remember that books can be borrowed through interlibrary loan - you can go on the library computer, check out titles, order the books, and have them delivered to your local library, where you can also return them. You don't have to go chasing all over the state. You can preview the books, and choose for your child the ones you think are relevant/appropriate.
There will also be 2 houses for your child to live in - that should be presented as a positive, regardless of how you feel about your boyfriend. Hopefully you can work together to maintain the same rules and structure and routines in both houses - that is key for the children.
Good luck - it's not easy and your feelings will change over time. You will experience sadness, anger, jealousy, frustration, loss and so on - be sure you have someone to talk to so you don't vent your frustrations in front of your children. Hang in there.