There are lots of factors to consider, some objective like whether they have the appropriate qualifications and PI cover and some highly subjective such as whether you feel a good personal chemistry or whether their previous designs are in a style that you like.
An architectural designer’s role can range from having minimal input outside of draughting up the plans for planning purposes to that of semi project managing the build and involved in virtually every aspect. Some designers have multi-discliplinary skills, which can add value and save costs.
Key to a healthy relationship is understanding precisely what their role and responsibilities are and having everything agreed, recorded and documented from the start so that both parties are clear what is expected.
Whilst choosing a design from a published book is still possible, the changes to building regulation means that you will still need a professional to comply with them so these are best used for layout inspiration.
Your architectural designer will advise you on the planning guidelines for your area, so it is very important to get this advice before you get too fixated on a design which ultimately might not be suitable for the site.