Firstly, good luck! I hope all goes well with the birth. Its a very exciting time but also nerve wracking knowing how your toddler will cope. I'm sure at first they'll be over the moon with their new baby, but you are right to be thinking about the next step beyond.
Everyone is different. I'm sure Gina was fine for some whilst not for others. I found her routines very restrictive, so ditched her books once number 2 came along. My best advice is to fit baby around toddler. For most people I know this has worked well. Your new baby, as long as it is fed, changed and has the opportunity to nap in the pram, will be content and happy enough with the opportunity to get out and about, see new faces and generally watch their older brother/sister have fun with you.
That said there is always something more we can learn from a good book if we have the time to read it
My third child is now 17 months, and my husband recently bought Toddler Taming by Dr Christopher Green. I wish I'd bought this one years ago! With hindsight its easy to see how this book could have made my life so much easier. It doesn't go into detail on juggling several toddlers but it will give you an insight into how your toddler might be thinking and helps you find ways to address this. If you have been an avid Gina fan to date, this one might give you a new perspective.