Jane, though wireless telecoms might sound like a “niche” to many people, the aspects of this domain are broad. The documentation that you need to look for will depend not only on the subject that you are interested in, but also from what you already know. As a general practice in this domain, I recommend that you borrow a book (maybe you have telecom engineers in your environment who can lend one) or buy one… though the price seems high (60-100 euro) at the beginning, but it’s worth!
To me, working on KPIs performance for Radio Access (Air and Abis interface), I find very important some concepts like speech coding (A and Mu laws, or let’s call it logarithmic compression) and source coding (mainly the error-correcting coding which allows to “fix” your frames even if some bit errors occur), even though it’s not needed for my everyday tasks… but it helps a lot to understand better what you will learn or do in the future (if you work in telecoms).