Try to read the manual more actively. In other words, do not only stare at the words, but become active. For instance, consider some of the following techniques:
Start by prereading the manual, i.e., do not start by reading the manual cover to cover, but start by reading the title of the manual, the publisher's blurp (if there is any), the preface or introduction, and then study the table of contents. Then start reading parts of the sections that you discovered are most relevant to you (summary paragraphs at the beginning or end of chapters are especially good to read when prereading).
Ask the manual questions, which you then answer by looking the answers up in the manual.
Write an outline of the manual as you read. Note that the outline does not need to coincide with the table of contents and can go into more depth.
Underline important words and sentences.
Write brief summaries of sections or paragraphs.
Sketch a mind map about what you are reading while you are reading it.
The above techniques apply to any expository work (as opposed to novels, for instance) you read, and thus apply to manuals as well.
If you want more information look for books on, or google, the subject of "active reading".