You do not need any specific books for that. All you need is, know what is the traffic that you are expecting. calculate the erlangs and based on erlangs and using Erlang B calculator, comeup with teh number of ckts and that will provide you teh number of E1s. IF these E1s are R2 (then no issue) if these are SS7 based put one pair of SS7 signaling likns for every 30-35 E1s