I was wondering if anyone had ever read any books or found any other sources that said what the Breen really look like under their suits.
According to Worf, no outsider had ever seen what a Breen looked like under their refrigeration suits and lived. Even after joining the Dominion, their appearance was unknown to their new allies. Weyoun couldn't help but wonder what they looked like. (DS9: "Indiscretion", "'Til Death Do Us Part", "What You Leave Behind")
So, I don't believe that there is any canonical information on what the Breen actually look like. "Zero Sum Game" by David Mack, provides an interesting explanation:
It was not until 2382 that definitive information was obtained relating to the Breen and their biology. Initially it was believed that the Breen were a single race; however, as detailed by Sarina Douglas and Julian Bashir, the name Breen is a name taken on by the member species of the Breen Confederacy. It has been claimed that the individual races (the Amoniri, Fenrisal, Paclu, Silwaan) take on the singular name and the concealing uniforms in order to prevent any biases and prejudices to interfere with the development of their society. The uniform design has been designed to support the distinctive features of the individual races. (ST - Typhon Pact novel: Zero Sum Game)
I recommend checking out Memory Beta's page on the Breen for a pretty detailed account of theories and information on their appearance.
There are times when it's better that a mystery not be revealed. The perfect example, I think, of this are the Tholians. For years they were a race often spoken of but never seen in a Trek tv episode. While some of the novelizations and books contained descriptions and drawings, they remained for most a mystery of Trek tv. When they finally were shown in a couple of the final episodes of Enterprise, the "real life" visualization was an extremely disappointing letdown.
So some things are really better left to the imagination.
According to Worf, nobody ever saw what's beneath a Breen helmet and lived to tell about it so that is a clear indication that both Dukat and Kira, either they found spare suits or the suits they stole from the Breen soldiers, they had other masks underneath those helmets so the Breen soldiers were covered and both Dukat and Kira never saw what the Breen looked like. The design of the Breen helmet was intentional on account of Ira Steven Behr, DS9 producer who said that they weren't in the mood to come up on a new alien race so they thought of covering the Breen up because they live in the cold and the helmet design indicates that the Breen are that close to arctic wolf. David Mack's Novel is not canon, In Canon DS9 there has never been a clear answer to that question but only suggestions mostly asked by Weyoun and Damar when they shared times with the Breen in the last days of the Dominion War.
I just read in Star Trek Online's news page from a Benzite
The Starfleet officers in the brig slowly blinked to wakefulness, sitting up and re-examining their surroundings. The Breen at the security console nodded, dropped the force field, then pulled off its helmet.
In response, the Benzite said, “I thought the Breen were supposed to be amorphous.”
Daniels cracked a smile. “Come on. I can’t do this jailbreak alone.” He gestured and the other two Breen took up positions – one moved to guard the door, while the other stepped into the cell and started distributing small, thin plastic strips imprinted with microcircuitry. “Attach these to your uniforms,” said Daniels. “We’re getting you back home.”
So they may not have a solid body, as human do, but maybe more like Alasomorphs or the Founders of the Dominion. this last could help to explain why they allied with the Dominion during the Dominion War.