Generally I am not a fan of opening books, but these two are highly recommended to players in the, say, 1000 up to 1800 or 1900 range. Both books were originally published in 1997 making them, as opening books go, ancient, but that's OK because they are BASIC books.
Both books are guides to the openings and cover popular modern openings. Many opening books have a bewildering array of variations, but in these two books Kallai concentrates on developing a solid understanding of the ideas of each opening with main thrust being that you need a to establish a sound and promising position out of the opening which will help in formulating a viable middlegame strategy. In this book Kallai teaches the basics of e4 openings. For 1 d4, 1 c4, and 1 Nf3 you need the companion book.
Also published in 1997, this book covers 1 d4, 1 c4, and 1 Nf3, as well as some of the lesser known alternatives to 1 e4. As in Basic Chess Openings Kallai describes what you are trying to achieve with each opening.
The books are published by Cadogan which has a reputation for poor bindings that allow the pages to fall out after moderate use, so just be aware of that.