I'd like to have some way to differentiate between yǐqián (以前) and zhīqián (之前), in order to help with memorising them both. Referring to Modern Mandarin Grammar (Claudia Ross, Routledge 2006), it seems to suggest that the only point of differentiation is that zhīqián is more literary. Is this the only difference? If not, what do I need to know?
As a native Chinese speaker, I agree with what you found in the book. Two words are almost identical except that Zhiqian is more literary. But, on a second thought, it seems that zhiqian is similar to "earlier" while yiqian is similar to "used to". For example:
In its meaning of of "used to", 以前 can be used to start a sentence, while 之前 cannot:
很遗憾，越来越多的说中文的人把“之前”当作“以前”用。It is very bad more and more Chinese people are using “之前” where they should use “以前”.
I'm not a native speaker, but it seems to me that in some cases the usage of these two terms is different and not interchangeable.
For example, I think you could say 我三岁之前，我只喝牛奶 (or 只能喝牛奶), to have the meaning "Before I was three", but it would sound a little odd to say 我三岁以前，我只能喝牛奶, wouldn't it?
I think both terms can mean "in the past", but it seems to me that if you are referring to an event that occurred prior to some other event in the past, 之前 sounds better (but for something that occurs before something else in the future, both 之前 and 以前 are OK - for example, 我去欧洲之前，我需要学习法语 or 我去欧洲以前，我需要学习法语 (although personally I still like the sound of 之前 better than 以前 in this sentence too).
Please correct me if I'm wrong.