After going through all of the effort to teach your baby to sleep well, it can be unsettling and unnerving if your toddler starts to cry at nap time.
While solving a baby's nap issues can be tricky, it is more straight forward than solving a toddler's nap issues. The older children get, the more complex and individual they become. In this post, I discuss various reasons a toddler might start crying before nap time and how to respond to each scenario.
For this post, I am assuming your toddler has been a good sleeper in the past and was taught and expected to sleep independently already. The crying is different than how things have been in life for your toddler up to this point.
Toddler is Crying Because of a Sleep Regression
Since you have made it this far into your parenting journey, a sleep regression is nothing new to you. You know all about sleep regressions and what they mean. This is when your toddler suddenly stops sleeping well and seemingly for no reason. If this is a literal regression, there is nothing you can do but wait it out.
At this point you should know your toddler well enough to have a good idea of how to respond and how not to respond during a sleep regression. Does your toddler need to just blow off some steam for a minute before settling down? If yes, just let her be. Does your toddler tend to do better if you go in and address her somewhat? If yes, go in.
This is of course my first recommendation for getting your baby to have great sleep habits. When I had my first two children, this was the only thing I had in my arsenal. While the book does not talk much at all about sleep training, it lays a great foundation for a great daily routine to allow for healthy sleep. 
This is my favorite book for just overloading you on information on sleep. The importance of sleep, the mechanics of sleep, the cycles of sleep, the patterns of sleep...it is all there. He also talks about different methods to sleep train and solve sleep issues. I first read this when my third child was a baby and I have loved it ever since. 
This is a two-book recommendation. We have 
This book doesn't so much give sleep training help, but it is a must-own to have in your library when you are sleep training a baby. This book helps you know when there are developmental leaps. There are ages a baby just isn't going to sleep well. There are ages sleep regressions happen, no matter how great of a sleeper the baby is. Having this knowledge helps you approach sleep training the best way you can.
Of course everyone knows about the Ferber method of sleep training. This is not a book I read while I had babies, so I offer this up as a consideration for you because it is wildly popular and used by so many. I am all about knowledge and getting all you can, especially when it comes to sleep training a baby. What works for one will not work for another, so the more tools you have in your back pocket, the more likely you will reach baby sleep success. 
This is a book I have not read at all. I have heard the ideas are very compatible with Babywise, but that it is much more hardcore than Babywise. From what I understand, it is more of a "do exactly this" book than a "figure out the best option for your individual baby" book. If that appeals to you, give this book a try. 
This is another one I have not read. I am adding it because it is a common book people turn to for sleep advice. If you have exhausted the books above, look into this one and see if it will offer you further insight. 
