The English Opening is very popular nowadays, but 1.c4 e5! is a serious challenge and not to everyone's taste. Here comes 1.Nf3, with the idea to transfer the game into the English with c2-c4 to follow. The only hurdle on that path is 1...d5. Then White is at a crossroads. 2.c4 allows 2...d4 3.b4, while 2.e3 is an attempt to deny ...d4. Black can revive the threat with 2...c5, and again, White has a choice. 3.c4 d4 4.b4! leads to the Blumenfeld with reversed colours. Or he can continue to fight against ...d4 with 3.b3. White's decisions on the second and third move depends on the way he wants to meet the Queen's Gambit and the Slav/Meran/Chebanenko.
Semkov covers in detail the different options, and also devotes several chapters on 1.Nf3 d5 2.e3 Nf6 3.c4 c6 and 3...e6 4.b3! (avoiding the innocuous version of the QGA after 4.Nc3 dxc4). There is another chapter on 1.Nf3 d5 2.e3 Nf6 3.c4 dxc4, where the absence of Nc3 allows White to get some pressure by refraining from d2-d4.