The world is alrerady infested with people at sessions singing songs with their heads buried in their ring-binders full of words down-loaded from the internet. I dread to think what will happen if people all learn to sing at sight and start singing straight off the dotrs in public!
Having got that grumpy old man bit off my chest, sight reading is an invaluable skill, and well worth acquiring.I dont think you need special books. Start off with any song book, pick a song you know the tune of(hopefully what you remember will be more or less the same as the tune in the book...these are folksongs and may well vary). Sing it, and watch the notes go up and down . See how the process works.After you get the idea, progress to a tune you've heard, but only vaguely remember. And so. Pretty soon, you will be able do it from scratch on a tune you dont know. Some, especially with accidentals or long leaps, are harder than others. Tunes that progress by single scale steps are the easy ones, cut your teeth on those.
When you've done it, check that you have it right by playing it on the piano or guitar or whatever you are familiar with. Does it sound the same?