(When I happened to wish for a Siddur the other day in another thread, I was sure there already was a formal Suggestion for one, but it seems not, so I thought I'd better add one.)
We need a minimum of one [Hebrew/English] Siddur, as complete as possible, but preferably several, reflecting a wide variety of Jewish traditions. I leave the selection to others, better suited.
We also need books on Jewish prayer. Unless someone more knowledgeable has better ideas, I suggest
And we also still need at least one Jewish lectionary, but again preferably several, reflecting various traditions.