I was in a similar situation a few years ago...
I'm not sure that I would have been able to do it with just books though.
I took Spanish classes at college and ended up majoring in Spanish. My dd and I traveled a bunch- Mexico, Ecuador, Peru, Guatemala. She's enrolled in a Spanish-immersion charter school now (she's almost 10) and is fairly fluent, especially with speaking and reading.
When she was younger we only watched movies or TV in Spanish, which was surprisingly helpful. We're not so good about doing that now..
I found that the first time we went traveling when she was 3 she didn't seem to "pick up" all that much language. We spent 3 months in Mexico and she went to a Spanish-only preschool a couple days a week for a few hours.
The next year we were in South America for 5 months. By the time we came home she was having major conversations without any help!
She didn't go to school at all this time.
The following year we went to Guatemala for about 6 weeks. Again, no school for her.
It's been a few years now since we've been able to travel and I was worried that she'd forget everything. She's doing fine though- I think that school has really helped her retain what she knew- plus she's learned new stuff!
Good luck mama!