Defamation would only be a valid concern if the information was false and put the company or companies in a bad light.
You can use the trademarks of products under something called "fair use."
The Lanham Act permits a non-owner of a registered trademark to make "fair use" or "nominative use" of a trademark under certain circumstances without obtaining permission from the mark's owner. Fair use or nominative use may be recognized in those instances where a reader of a given work is clearly able to understand that the use of the trademark does not suggest sponsorship or association with the trademark owner's product or services and therefore is not being used in a manner to confuse the reader. One example of fair use is when the trademarks are used in a fictional work, such as "She took out a Kleenex."
Trademark law also permits an author of a non-fiction work to include content that is favorable and/or critical of a trademark owner's products or services. In this type of work the author should only use the trademark to describe or identify the trademark owner's product or service and should be careful not to confuse the reader as to the actual provider of the trademark owner's products or services. This is the type of use you are probably going to have.
An author's use of a trademark for the above-referenced situations should be considered a non-confusing "nominative use" when it meets the following requirements: (1) the trademark owner's product or service must be one that is not readily identifiable without the use of the trademark; (2) the author only uses as much of the trademark as is reasonably necessary to identify the trademark owner's products or services; and (3) the author does nothing that would, in conjunction with the trademark, suggest to the reader sponsorship or endorsement by the trademark owner.
If there is any question of your using the names of other products, you will claim that you had the right to use the names under the law.
The fair use defense if it is to be successful must meet the following requirements: (1) the author's use of the mark must accurately describe the trademark owner's product or service; (2) the author must use the mark in a non-trademark manner and not as a source identifier of the author's work; and (3) the author's use must be in good faith.
One difficulty with the fair use or nominative use defense is the lack of legal certainty that can be relied upon.
Permission is not needed to use brand names in fiction as long as the use expressly refers to the trademark owner and the reader will not be confused about whether the use is one of sponsorship or endorsement. Incidental use of trademark should fall under the category of "fair use".
Think about publications such as "Consumer Reports." They use trademarks on every page--names such as KitchenAide, Harley Davidson, and so on. They compare the good and the bad, and they have every legal right to use these names.