I think the best way is `awk` and `gawk`.
##awk

 awk -F "([.] )|( / )" '/^[0-9]{1,3}[.]/{x="F"$1"("$2").txt";}{print >x;}' I_Ching_Wilhelm_Translation.txt

`-F` will specify fields seperator for each line. It is a regex, here we use multiple seperators: `". "` and `" / "`. Thus a line like `1. Ch'ien / The Creative` will be split into 3 fields: `1` `Ch'ien` and `The Creative`. Later we can refer to these fields with `$n`. `$0` is the entire line.

We then tell the awk to match the lines with pattern `^[0-9]{1,3}[.]` If there is match we then assign value to `x`. The value x will be used as file name for `print` operation. In this example we use `"F"$1"("$2").txt"` so the line `1. Ch'ien / The Creative` gives a filename `F1(Ch'ien).txt`

##gawk

In gawk, we can also access captured group. So we can simplify the command to:

 gawk 'match($0, /^([0-9]{1,3})[.] (.*) \/ (.*)$/, ary){x="F"ary[1]"("ary[2]")";}{print >x;}' I_Ching_Wilhelm_Translation.txt

here we use `match` the capture the groups and put them into variable list `ary`. `$0` is the entire line. `ary[0]` is everything matched. `ary[1...n]` is each group.

##perl
We can also do it with perl:

 perl -ne 'if(/^([0-9]{1,3})[.] (.*) \/ (.*)$/) {close F; open F, ">", sprintf("F$1($2).txt");} print F' I_Ching_Wilhelm_Translation.txt


Results:

 > ls F*
 F10(Lü).txt F22(Pi).txt F34(Ta Chuang).txt F46(Shêng).txt F58(Tui).txt
 F11(T'ai).txt F23(Po).txt F35(Chin).txt F47(K'un).txt F59(Huan).txt
 F12(P'i).txt F24(Fu).txt F36(Ming I).txt F48(Ching).txt F5(Hsü).txt
 F13(T'ung Jên).txt F25(Wu Wang).txt F37(Chia Jên).txt F49(Ko).txt F60(Chieh).txt
 F14(Ta Yu).txt F26(Ta Ch'u).txt F38(K'uei).txt F4(Mêng).txt F61(Chung Fu).txt
 F15(Ch'ien).txt F27(I).txt F39(Chien).txt F50(Ting).txt F62(Hsiao Kuo).txt
 F16(Yü).txt F28(Ta Kuo).txt F3(Chun).txt F51(Chên).txt F63(Chi Chi).txt
 F17(Sui).txt F29(K'an).txt F40(Hsieh).txt F52(Kên).txt F64(Wei Chi).txt
 F18(Ku).txt F2(K'un).txt F41(Sun).txt F53(Chien).txt F6(Sung).txt
 F19(Lin).txt F30(Li).txt F42(I).txt F54(Kuei Mei).txt F7(Shih).txt
 F1(Ch'ien).txt F31(Hsien).txt F43(Kuai).txt F55(Fêng).txt F8(Pi).txt
 F20(Kuan).txt F32(Hêng).txt F44(Kou).txt F56(Lü).txt F9(Hsiao Ch'u).txt
 F21(Shih Ho).txt F33(TUN).txt F45(Ts'ui).txt F57(Sun).txt


#how to get the example file:

 curl http://www2.unipr.it/~deyoung/I_Ching_Wilhelm_Translation.html|html2text -o I_Ching_Wilhelm_Translation.plain
 sed 's|^[[:blank:]]*||g' I_Ching_Wilhelm_Translation.plain > I_Ching_Wilhelm_Translation.txt