I was curious if when creating a 4e D&D character if I could use multiple books to do so. For example I want to make a Druid, can I pick 1 at will power from one book and another at will power from a different book. Say One from Heroes of the Forgotten Kingdoms and one from the book Primal Power?
All editions of D&D have a concept known as the “core” books, books that are always assumed to be in play, and supplement books, which were add-ons to that core. If nothing else, supplements would frequently refer back to those books such that it was impossible to use the supplement without including some material from that those books.
For practical reasons, supplements often do not refer to each other; authors cannot assume you have all of the other supplements, so anything they write referencing another makes that material worthless to those who don’t have that book. But with 4e, Wizards made the decision to call all (non-setting-specific) books “core,” that is, an assumed part of the game by default. Thus, according to Wizards, you should be able to go out and buy a brand-new book, create a character using it (and all the other books), and show up to a table with no problem.
So 4e definitely did have supplements reference other supplements far more than previous editions. Primal Power would be impossible to use without Player’s Handbook 2, since most of the primal classes are defined there.
This was a massive change from previous editions, in a sense, because previously the DM was always the arbiter of which books were allowed in a game. Adding a new book to a game was always a matter of discussion with the DM first. Note, however, that if a supplement was allowed, it was expected to combine (at the very least) with the core books. In most cases, it also could combine with any other allowed supplements, though a few groups did define limits on the number of books a single character could use. (On a personal note, I consider this a terrible practice that is significantly to the detriment of the game played.)
Ultimately, what Wizards does or doesn’t say only matters so much; groups still can and do limit the available books. Plenty of groups still give that authority entirely to the DM. Plenty of DMs still see it entirely as their right, and plenty of groups will back them up on this.
So to sum up:
Wizards says yes, everything is good to go and you can use any and all books you have.
Some groups disagree, and allow or ban books on a case-by-case basis. Many of these groups give this authority solely to the DM. This matches the expectations of previous editions.
Even if available books are limited by the group or DM, usually you can combine material from any allowed books as you like.
A few groups, however, specifically limit how many books any one character can use, even if all the books are allowed.
So ultimately you are going to have to check with the group that you play with to see how they feel.